if your cage is 72" long by 18 " tall with a total of 6" in rails ( borders ) than your 2 glass pieces will be roughly 12" x 36" . Use 1/4" minimum plexi. For a burn I would say use 1/4" polycarbonate. Much more expensive but burbs can push pretty hard. My guy is a beast and "tests " my glass daily. Also don't forget to reinforce your glass tracks. I glued and nailed mine down to be double safe. It doesn't matter how good the glass is if he can blow the tracks out.