I had a specific need, and I figured the expanding foam underneath the pink sheet would self-level, and it did. I also figured the underside would take the shape and fill the uneven spaces of the existing landscape. The foam did that too, to a certain extent. The top piece is very stable on the original layout.
I sprayed the underside of the pink circle with spray paint, which dissolved and roughed up the surface enough to insure that the expanding foam would adhere to it. I chose cooking oil over silicone spray for the plastic sheet, thinking paper-mache was pretty organic. The plastic slid right out from under the mountain after everything dried.
Drawbacks? - 4 cans of Crack-Filler coast me over 20 bucks.... and the plastic sheet keep air from getting to everything, so the foam expanded and cured unevenly, so I had to do a little sculpting on the underside when everything dried. I like to share what I'm working on, hoping my cohorts here (and anyone who reads my blog) might benefit from my experimentation.
For the right application, this Enterprise works pretty well.