I used to do this kind of stuff for all my reptiles. Yours looks ace! Love the elm bark.
When I did mine, I covered a backing piece of hardboard or something with expanding foam (the stuff that comes in the aerosol can) and pushed bark into the wet foam. When it sets it goes hard like polystyrene and I would coat the exposed foam in silicone and push dirt and sand into it to fill the gaps between the bark.
I used the fake modelling moss and lichen you can get in model railway shops to finish it off, with some dried mushrooms you can get on eBay for pennies.
I probably went a bit over the top, especially considering the fact that reptiles are just as happy with kitchen roll and toilet tubes as long as they can hide!
I thought about doing it for those guys who play wargames, like the little figures you can paint and stuff. They tend to buy most of their stuff and it costs a fortune. Maybe you should start pitching background ideas to them!
Here's a couple of mine, one with a fake rock wall, and one more jungle-esque with bark and foam
Hope you don't feel I've hijacked your thread! Don't get to show 'em off with people who do the same thing very often!