Last ones I made were 1/4 inch plywood I think, with 2x2's framing it. worked out nice except I used plexiglass for the doors and my guy could lean on them to bend them enough to pop them out of their tracks - and bingo, he's out. I ended up tacking some wood around the edges of the plexi so they didn't bend, but if/when I build more I'll probably just do glass. Plexi is too flexible, plus it scratches up pretty quickly.

Anyway overall the cages worked out nice, and it was pretty cheap (maybe $250 to build 2 8x3x2's). Only thing you might want to do is pick something besides the plywood for the bottom. I was using one of these cages for a Dumeril's monitor, I had pretty deep substrate and it was pretty moist in there - and the bottom started to warp & leaked a bit. So maybe try something more solid just for the bottom (melamine maybe?)