Hi, welcome to the forum...I have been building custom enclosures for years and years, along with decor, I used to sell them online and took custom orders, so I'm familiar with your issue, lol. I think I can help you out...
For an enclosure that is 4'x2'x2' you absolutely must have a T5 strength UVB tube inside a fixture that is rated for a T5 tube; The most crucial detail you must make sure of when you purchase both your tube and fixture is that the fixture is not only rated for AT LEAST the wattage of the UVB tube you're planning on buying or have already bought, but that it also contains a Reflector inside it. With such a large enclosure you must have the
UVB light Reflected throughout the entire enclosure, not just directly below the UVB tube itself.
A 36" UVB tube is perfect for a 48" long enclosure, that is usually the length that I install in the 4' enclosures I build. I believe that the 36" Arcadia 12% T5 UVB tube is 24 watts, so the fixture you buy must be 36" long and rated for at least 24 watts, and have a Reflector.
As already mentioned, the SunBlaster brand of T5 tube fixtures are excellent, they all contain a built-in, fold-out Reflector, and they also all come with several different sets of mounting hardware, so you can mount it to a flat surface, like the ceiling of the wooden enclosure, as well as the hardware to hang them. Here's the link to the 36" SunBlaster T5 fixture with built-in reflector, if you look under "Other Buying Options" up at the top, there is one seller who is selling this brand new for $43.77 shipped. That's a great deal, I think I paid that much for my 24" version a few years ago...
https://www.amazon.com/Sunblaster-904297-NanoTech-Fixture-Reflector/dp/B00AKKUD3Y/ref=pd_rhf_dp_s_cp_0_6?_encoding=UTF8&pd_rd_i=B00AKKUD3Y&pd_rd_r=JRH4RV1BY1GX6M84YN95&pd_rd_w=Ci5ms&pd_rd_wg=nloq1&psc=1&refRID=JRH4RV1BY1GX6M84YN95
There are a couple of 36" T5 Fixtures that are a bit cheaper, around $33 shipped on Amazon.com, but they don't include a Reflector, nor do they have a place to put a Reflector in the fixture, so you'd have to buy a separate, 36" Clip-On Reflector to attach behind/around the UVB tube, which would cost you more than the $43.77 shipped for the SunBlaster one with the fold-out Reflector. So definitely the SunBlaster above is the cheapest way to go with the best fixture/reflector combo.