RSS have you seen my posts on making the mud/clay wall background? I can go as high as I want with it. I've currently gone as high as 24" and used it to secure wood to the back wall and no worries about collapse.
My standard mix for enough to do a 10 gallon tank (+ a bit extra) is:
3 qts of all clay kitty litter (red bag of Special Kitty brand generic liter at walmart)
3 qts of sphagnum peat moss
1 qt of shredded LFS
1 gallon of water
Mix with a drill and "Jiffy Mixer" attachment
If you wanted to use sand or grit all you would have to do is add a bit more water. Basically the clay just binds it all and makes it tacky, while mixing it up if you can take a handful of material and squish it unsupported on the side of you pail and it doesn't slide down it's ready to use. If it's too wet and slides, just add more peat to dry it up a bit. Once it's installed mist it one or more times a day and you will have an all organic background that can be sculpted/shaped in various ways and holds just enough moisture for lush plant growth. I've been doing this in all my tanks, I'll never mess with silicone and spray foam again! Plants seem to love it.
Here's my first clay background in one of the vampire crab tanks, try to find the
Cephalotus cutting...LOL I am also using the background clay mix as the actual substrate in this tank too and it's working OK so far. This tank has been up and running for about 6-8 weeks and the clay background has reached a sort of equilibrium in moisture, it's about the same wetness as wet wood but is still malleable if I were to want to change something. Perfect for epiphytes to grow against and other kinds of plants can bury their roots into it. The crabs have both dug burrows here and there on the vertical background with no loss to structural integrity. To plant you just poke a hole and stick a cutting in or stuff it in the moss layer and let it find it's own way. Since the clay is non toxic I could do the side walls of the tank to gain more planting space at any time without moving out the crabs or plants.