I do mine exactly as Mike does. The only difference is I pack coco fiber or moss on top of it. The kitty litter option although really cheap does look rather gray and I prefer the brown look. Its biggest flaw is that it is extremely heavy even with a thin layer. It can also slide if you have it too thick in areas just because of the weight. I would have to agree with Mike and suggest doing a foam and clay combo on a 40gal. I think if you did it all clay you would need three people to move it. I did three sides of a 20 and I can barely budge it. They do make different color clays you can get at craft stores that might give a better look. I used clay in all my small Exos. You can see them in my photo album. The best thing about it is you don't have to wait after you're done. You can put the frogs right in.