I've used plate glass before that i had cut at a glass company. You can use any glass as a top, but it should be thicker than your average replacement window glass. .. You can also use clear acrylic and cut that but is needs to be pretty thick or it will warp from the heat... sometimes its just easier and cheaper to order a replacement glass.. Drsfostersmith.com sells alot of them...might want to check there. http://www.drsfostersmith.com/fish-supplies/pr/c/3578
hth,
al