Howdy
Chull, welcome to the thread!
Actually, silicone
may work, depending on the size of the pieces on the backwall - the backwall of my 'cube is basically acrylic/abs plastic, and I was told that silicone wouldn't hold to the slick surface, but almost 10 months later it is still sticking just fine. You can help adhesion by sanding the side that will have the silicone. The silicone also makes a seal so that detritus doesn't end up stuck behind. A good idea, IMO, even if you aren't counting on the silicone holding the rock up by itself.
But instead of acrylic, which will be a lot more labor intensive, how about egg crate? Cement will stick to it with little to no prep, and you can make ledge supports and tie it all in together rather easily.
I personally don't like the foam alternatives. I've seen too many threads where there were later problems as the foam ages and starts to get weird and flaky. And then you have crabs and the like that I've heard like to pick at the foam for fun...
And
everyone else 
I
did spend over an hour last night doing the write-up for the MRS, but when I hit the Submit button, I got a message saying I had used too many images, to go back and fix it, and of course, when I hit back, everything I had spent that time writing was gone. So I will re-write it and submit it later on today.
In the meantime, how about helping me choose my rock?
This is what I came up with - 3 pieces total. I used a 16oz bottle in the photo's so people would get an idea of the scale I'm working on here. Tiny rocks are tough to make
None of them suck, and I do have a favorite, but want to see if you folks agree. I'm still trying to figure out how to photograph white rock consistently - tried something different with Choice 3 and used no flash, and edited the colors after the fact. I did get more detail with that method, I think...
Front and side pic's of each rock:
Choice 1:
And the side view:
Choice 2:
And the side view:
Choice 3:
And the side view:
So which do you guys think?