My current grow closet isn't really an actual closet, rather it's a small area of unfinished basement originally intended for storage. Instead of adding sheetrock & floating/painting, I opted for the plastic light-block tarps you can get at just about any local grower's supply shop. I then installed some of that thin bubble-like, mirror-ish insulation, followed by the tarp. My growspace doesn't fill the physical space entirely, so the outermost "4th wall" is only the tarp with one of those light-blocking zippers (again, the grow shop) for access in/out and no worries of light pollution. Maybe this helps with more ideas around perfecting your available space as best can be. Oh, and copious amounts of white duct tape to seal everything up nicely!