I may be too late here, but the brand of tube fixture makes absolutely no difference, it's the "Wattage Rating" of the fixture compared to the UVB tube you're using. So if you're using a Reptisun 10.0 T5HO UVb tube, which if he has MBD is definitely your best option as he needs the T5 strength
UVB light, but you're using a tube fixture only rated for a T8 tube, this is your issue...
The Reptisun 10.0 T5HO UVB tubes are I believe 24 watts, while the T8 UVb tubes are only 17 watts, and most all T8-rated tube fixtures are only good to either 15 or 17 watts at the very most. So this is your issue if that's the case...Now you can go in reverse and use a T5-rated fixture to house a T8 UVB tube that is a lesser wattage, just not in the other direction.
As the Brand of Fixture you use makes absolutely no difference whatsoever, and the Reptisun brand tube fixtures are horribly expensive in pet stores, my advice is to go on to Amazon.com and search for whatever UVB tube you have, for example search for "22" Reptisun 10.0 T5HO UVB tube" and then when the tubes that are for sale pop up, scroll to the bottom of the page to the section that shows the "People also bought these" section, and you'll see all the different tube fixtures that are 24" long and are rated for a T5 UVB tube (most of the fixtures on Amazon that are rated for T5 UVB tubes are actually rated up to at least 26 watts, so they can also house the Arcadia 12% T5 UVB tubes). Click on one and then all of the hundreds of options they have will pop up. Again, the brand makes no difference, the only thing you need to be sure of is that the fixture you choose is rated for a T5 UVB tube and up to at least the wattage of your tube or higher, and that the fixture includes a Metal Reflector, or that you order a separate Slip-On Reflector that is the length of the tube, not the fixture (only applies to the Reptisun 10.0 T5HO UVB tubes, where a 22" tube fits into a 24" fixture, etc.).
I've been using only Reptisun 10.0 T5HO UVB tubes for years and I've ordered several different 18", 24", and 36" T5-rated tube fixtures on Amazon.com for them, I probably have 10 T5-rated tube fixtures that I've ordered from Amazon.com, all different brands (none of them are Reptisun or Arcadia brand), and I think the most I've paid for a single fixture was $30. I recently (3 months ago) ordered 3 new 24" T5-rated fixtures for new custom enclosures I built for people, all 3 had Metal Reflectors included (I chose the absolute cheapest 24" T5-rated tube fixture that included a built-in Metal Reflector) and I paid $24 for each fixture...