Yeah, so it does sound like he's too hot, and this combined with only having him for a few days will certainly effect his appetite. Relocation stress can actually last up to 2 weeks before you need to be alarmed, and that's with a normal baby going to it's new home. With a rescue that has had a difficult life, it can take much longer. With a 75 gallon tank, which is a great size for him for now, that 160 PowerSun MVB should still be enough during the day, but here's the problem with MVB lights, at least in my opinion, some people love them, some have tried them and agreed with me. But if you're building him a large, custom enclosure then I'd ask you to consider ditching the MVB light and going to a long T5 strength UVB tube in a long tube fixture with a reflector behind the tube (I have been building custom reptile enclosures for years, I used to build them to spec for people and made quite a bit of money doing so). Anyway, here's why I suggest getting rid of the MVB for your large, custom enclosure and going to a T5 UVB tube and separate, bright-white colored Basking Bulb:
First of all, large custom enclosures made of either wood or especially if you're going the polystyrene route, hold heat much, much better than glass tanks with mesh lids. Most people who go from a glass tank with a mesh lid into a much larger, custom enclosure with a solid top are shocked to find that even though they are going to a much larger enclosure, they actually need to go down in Basking Bulb wattage, because the temps are far hotter inside the larger, wooden enclosure than in the smaller, glass tanks with mesh lids.
The main reason that I suggest not using an MVB light (this connects to the above reason of larger, custom enclosure holding in heat much better than glass tanks do) is that YOU CANNOT SEPARATE THE UVB/UVA LIGHT FROM THE HEAT WHEN USING AN MVB LIGHT, AND BECAUSE OF THIS IT MAKES IT VERY, VERY DIFFICULT TO GET THE BASKING SPOT(S) WITHIN THE CORRECT DISTANCES FROM THE
UVB LIGHT AND STILL GET THE BASKING SPOT SURFACE TEMPERATURE WITHIN THE CORRECT RANGES. I absolutely hate not being able to independently control the heat light/temperature from the
UVB light. Say you get your MVB within the correct distance from the basking spot/platform, but the Basking Spot Surface Temperature reads 120 degrees...You cannot simply raise the MVB up and away from the Basking Spot/Platform to lower the Basking spot temperature, because then the UVB will be too far away and your dragon won't get adequate UVB/UVA light. That drives me mad.
The other thing about a large, custom enclosure is that the MVB lights do not reflect throughout the tank even close to as well as a long, T5 strength UVB tube with a reflector behind it will. I have 2 custom enclosures that I built and kept for my own dragons, one is 4'x3'x2' and the other is 4'x2'x2', and each of them have a 34" Reptisun 10.0 T5HO UVB tube in a 36" SunBlaster NanoTech T5 Hood with fold-out reflector, and the UVB tubes cover not only the entire Hot Side of the enclosures, but also almost the entire Cool Sides too (I also have a 2-bulb, adjustable light fixture on each side of the enclosure ceiling and right alongside the UVB tube on the Hot Side). So they get very intense, strong UVB/UVA light directly underneath the tube, but they are also getting more than adequate UVB/UVA light reflected to all parts of the enclosure, so no matter where they are inside the enclosure (except for under one of the built-in platforms or caves) they are getting very strong UVB/UVA light. If you put an MVB light over the Hot Side of a large, custom enclosure, even if it's mounted to the ceiling in a dome fixture with a round reflector, it's not getting adequate UVB/UVA light really to anywhere in the enclosure except directly underneath the MVB light itself.
Anyway, I'm intrigued that you're making a custom enclosure entirely out of polystyrene, I've done several of these, in most of them I used several coats of ModPodge all over the polystyrene inside and out to harden it. This makes the polystyrene as durable inside and out, specifically on the exterior, so that the enclosure will not get damaged when it's moved around. Several coats of ModPodge makes the polystyrene every bit as durable as 3/4" plywood or melamine board, and it's completely sandable so you can paint the exterior however you want to. I use small sized wall anchors to attach the pieces of polystyrene together after they have already gotten several coats of ModPodge on them to harden them (the wall anchors that come in a pack with the plastic, spiral anchors that a metal screw goes through). So I will drill the holes into the hardened polystyrene wherever I want to put an anchor, then I'll put the anchor/screw into the hole. I drill all the holes and insert all the wall anchors to the point that the all of the enclosure is put together, then I take all the wall anchors/screws out, and use a hot glue gun to run a bead of glue on each connecting piece and then a little in each hole, and then screw the wall anchors/screws back in, thus permanently connecting the pieces (obviously I attached the top last so that I could do the custom interiors). The hot glue in the holes will permanently set the wall anchors/screws. Then you essentially have an enclosure that is just as hard/durable as a wooden enclosure, but that weighs a fraction of what a wood enclosure weighs. Then I use clear caulking to seal all of the wall seams, and use the sandable grout on the inside of the back wall and the sides, along with designing custom polystyrene backgrounds, basking platforms, caves, etc. I then put down permanent textured, slate tiles as the substrate on the floor, I usually use the stick-on ones from Lowes and then reinforce them to the floor of the enclosure with Hot Glue.
I highly, highly recommend that you NEVER USE CALCIUM SAND FOR ANY BEARDED DRAGON EVERY, NOR CRUSHED WALNUT SHELL SUBSTRATE, NOT EVER, EVER, EVER, EVER...Calcium Sand/Reptisand (same thing) is the second deadliest loose substrate to Bearded Dragons, only second to the Crushed Walnut Shells. Calcium Carbonate Sand when wet sticks together and forms basically concrete rocks that do not break down without physical force being applied to them (like a hammer). Bearded Dragons do not come from the typical, sandy deserts we think of when we think of deserts. They naturally come from the deserts of Australia, which if you Google Image Search you will see are hard, rocky terrain with vegetation intermixed throughout, there is very, very little sand or even loose substrate, with the exception of large rocks. And Bearded Dragons lick everything, it's a type of sense of touch for them, so even if you don't feed them on the Calcium Sand, they will be ingesting it every single day. I cannot tell you how many Bearded Dragons I have seen, both on this forum/other Dragon forums and at the reptile and bird rescue I have worked at for many years, either killed by Calcium Sand and Crushed Walnut Shells, or who developed severe bowel obstructions that required emergency surgery or the dragon would die.
I don't suggest ANY loose substrates for a Bearded Dragon, especially if you're using it simply because you like the way that it looks (as you're not going to be using it because it's what's best for the Dragon or because it's close to their natural environment), but if you are dead-set on using sand in your custom enclosure I can't stop you obviously, I've said my piece and realize that I'm only saying it because I don't want to see yet another dragon die so needlessly from such an easily preventable circumstance, and I don't want to have another Dragon owner blaming themselves because they knew they shouldn't have used Calcium Sand, Crushed Walnut Shells, Wood Chips, Bark, Rodent Bedding, etc. But if you're going to use sand in your custom enclosure regardless of what anyone says to you, I'd ask that you at least totally avoid any of the Calcium Sands or "ReptiSands" that are sold in pet stores, and that you simply go to Lowes or Home Depot and buy a 50 pound bag of Screen-Washed Play Sand instead. It is much more natural and doesn't clump together when it gets wet to form rock-hard cement balls inside your dragon's GI Tract.
Keep in mind the other issue with using ANY loose substrate for a Bearded Dragon, including the play sand, is that Dragons are already prone to Upper Respiratory Infections, Eye Infections, Skin/Scale Infection (keep in mind Yellow Fungus), etc. and ALL loose substrates are breeding grounds for all types of microbes, including tremendous amounts of bacteria, fungi, parasites, protozoa, etc. And they harbor these microbes in tremendous amounts as soon as you put them in their enclosure. Even if you immediately scoop out a poop or any food, such as insect parts, veggie, greens, and fruit bits, shed skin, etc., the microbes start infesting the loose substrates immediately. So you're putting your Dragon at risk for several chronic infections needlessly.
The textured, stick-on, slate tiles that they sell at Lowes and Home Depot (Lowes has way more options than Home Depot in-store, and they're cheap) are not only extremely easy to spot-clean, they keep their toe-nails trimmed naturally, and I think they look awesome inside any tank, but especially inside custom enclosures with custom polystyrene backgrounds that are grouted. Something to think about.
If you ever have any questions about building your polystyrene enclosure, feel free to PM me any time. They are rare to find, not many people build them because they don't think they will hold-up like wood, they expect them to just chip-away over time and to lose chunks out of them if you bump them against something or the dragon digs at them, etc. But I've built at least 6 of these that I can remember, and it's my experience that the key to their durability is getting enough layers of hardener on every piece of them, on all the edges, everywhere, so that you build-up a strong, hard outer shell (and down into the polystyrene, the hardener pretty much seeps the entire way through a 3/4" piece if you apply a coat on the top, bottom, and both side edges). This makes them every bit as strong and durable as 3/4" plywood, but they weigh a fraction of what a custom wooden enclosure weighs, and they also hold-in heat far better.
