Hey hey Im actually doing mine now hahaha.
What i did was got some expanda foam from the hardware shop.
Just sprayed it against the back wall how ever you want it. Have a few bottles around and have a practice first as well. This stuff is very versatile you can cut it sand it and paint it. I havnet tried melting it yet but yer havent had to
Then get some fine sand (clean) i then mixed some black non toxic acrylic paint with sand and small amount of pva.
That was the first three coats, the sand gets that natural look in and you want to really cover that foam up to.
Then go nuts with colours really.
Try using a dry brush or a sponge to get some texture happening.
After that use a substance called pondtite. You get it from most good hardware stores.
It will water proof the foam and paint keeping dangerous chemicals out.
Do a few coats i recommend about 3-4
Thats just how i did it.
Atm Im working my frog enclosure and my large snake enclosure. With the snake enclosure i just used Bondtite which is basically a less water resistant version on Pondtite (same makers).
Just search this as well on the forums theres countless ways of doing it.