You are a little ahead of me in defining your mission, but if I understand you correctly, think about this with me. I am in some very preliminary stages of a setup. I am almost sure to use a derivative of Gilles' background technique as described here. I have been pondering the thought of terracing a section of the bottom with the same material, possibly leaving "bowls" that will stabilize substrate?is it possible for the substrate to suport the rocks so there is adiquate space between them to put plants and actually embed the rock in a "mold"? I want to make (in the 2 back corners ) a side of a hill.
Am I following you correctly?