I've thought about making the frame out of a decent plywood, lining the interior with fiberglass wall panels, and using silicone caulk to seal the seams.

These are the fiberglass panels, they are 4'x8'x0.09":
http://www.lowes.com/pd_8566-293-FTSTF.1_0

http://www.lowes.com/pd_8566-21553-8566_0