For the background, I started with two MDF boards which I screwed together with metal angle brackets, to have an L-shape going around a corner, covering most of the back and one side of the enclosure (the enclosure of my dragon is 120 cm long and 80 cm deep, and this is about 110 cm long, 60 cm deep).
For some "boards" protruding from it, I then nailed syrofoam to the MDF boards (actually: piercing the MDF board with nails and putting the syrofoam on it), then covered them in construction foam to make a thicker, more sturdy shape and make everything holding well together.
I let the construction foam dry a few days; during that time it expands. Afterwards I cut it down a bit using a knife when it just was too round and looked too much like "icing on a cake", to give it more a stone-like shape. I then covered everything with a layer of tile glue (that's something similar to concrete, not similar to a normal glue) mixed with acrylic paint. The tile glue itself dries to a light grey, almost white surface. I added another layer of that tile glue mixed with acrylic paint and this time pressed sand firmly to the surface. Once dry - I just waited until the next weekend - I brushed off any sand that's loose.
With that it was finished. I glued the background into the enclosure using aquarium-grade silicone (a big "blob" on the back of the enclosure and firmly pressing it on). I then sealed it all around with the silicone, to make sure no insects can get behind that background.
I have built this before I got my dragon. As you already have your dragon: I would recommend when you glue in the background with the silicone, to house your dragon in something else temporary for 1 - 2 days to let it air out. I let the background itself air out two weeks, but also the silicone stinks and of course the dragon should not come into contact with the still liquid silicone. Maybe you can put your dragon, if you have no other tank available, in a tote or large box for a day or two, putting the top mesh of the enclosure on top to prevent escaping.
All materials can be found in any kind of hardware store. (I actually reused the styrofoam from when we got a fridge delivered as we moved right before, but also the styrofoam can be bought in a hardware store.)
This kind of background is very sturdy, I see no "wear and tear" from the claws, and I thing because of the materials automatically giving a stone-like, natural appearance it always turns out nice and one doesn't need to be especially "artsy" for that.
I have attached some images from when I built the background. Image 1 is with MDF, syrofoam and construction foam. Image 2 is with first layer of tile glue mixed with acrylic paint added. Image 3 is the finished surface now with a second layer of tile glue and paint with sand added, as it looks in the enclosure.