Great stuff is basically expanding foam insulation. It's been used primarily (in the pet hobby) for making bacgrounds for poison dart frogs, crested geckos, etc. It's great because it looks really cool, and it provides (for climbing animals) vertical space. I originally made the bacground because I wanted to put plants in my tank, which you would do by cutting a hole in the bacground and siliconing the plant's pot in place. Lucky for me I tested the uros in the tank BEFORE adding the plants and found out that it wouldn't work because they climb so much.
To make it, you buy the 'great stuff' foam, it comes in an aerosol type can at home depot or lowes. One can is about $7. For my 75 gallon tank, I used about one and two thirds of a can. You'll also need sand. I used a mixture of playsand and decomposed granite, which I also got at home depot, because it gives it a more rocky appearance. ( Poison dart frog people use coco fiber for it, and it turns out looking sort of tree bark-like.) If you don't mix the sand it sorta looks like playsand on elmers glue. Lay the tank or enclosure on it's back on a level surface and spray the foam on. You can't really spread it after spraying, so I just went in a pattern, making row after row of spray all the way down the length of the tank until it was all covered. You have to finish this step fairly quicly because it starts to set up rather fast. Next, you take the sand mixture, I used a dog bowl, and pour it over the great stuff. Make sure you cover every inch of it. Now you sit and wait. The foam will start to expand, and at some points it will expand so much that it will push all of the sand off of that area, so you have to be ready to dump a little more sand on it. Once the foam is no longer active, you have to leave it to set up, I left mine for about 3 hours. Then, you set the tank or enclosure upright, so that all of the loose sand comes off, and voila! You can then go back with silicone and glue some sand over and bubbles that broke and left the foam bare, or any spots that just need touch ups. Let it off-gas for at least 48 hours. It cures in 8 but I like to be safe. Who knows how long the silicone takes to off gas anyhow. Then you decorate your tank as you like. Here's a pic of what it looks like in mine.
At first I was really worried they might dig into it and eat a piece of it, but it's been in with them for probably 6 months and put up to a lot of digging, scratching, and climbing. I've been pretty impressed and pleased with it. The big plus is that if it does become a problem it's fairly easy to remove, too. just pull off what you can by hand and scrape the rest down with a putty knife or similar tool. Probably would be difficult to remove from a wooden enclosure, though.
