How large of an enclosure do you need? It might be faster and cheaper to get a tub with a flat bottom shipped than to get everything you need to replace the floor in the foot locker shipped. Plus how are you going to seal the floor, caulk? And where would the snake stay while the caulk cures?

You said you used Lexan for the windows. Lexan is polycarbonate which blocks a lot of IR radiation depending on thickness and type, so it wouldn't work well with a CHE anyway. If you can replace those with thin plexiglass a CHE or DHP would work as thin plexiglass transmits most IR.