Use more rock than foam if at all possible. The goal is to have the majority of your wall rock with just foam to fill in the gaps. If you've got too much foam you'll have problems with buoyancy, though you'll likely have to silicone the rock wall to the back of your tank anyway. I'd use the Great Stuff foam at HD/Lowes instead of the black pond foam...it's cheaper and IMO matches the rock better, especially when you're coating it with epoxy and sand. Bear in mind that the Great Stuff foam expands A LOT. I knew that it expanded some but then the next morning when I went to look at the rock wall it expanded at least 2x more than when it was partially dry the night before.
Definitely use as much rock as possible, using the foam to fill in the gaps between the rock. Using dead rock for this project is fine. Zip tie the rock to the egg crate before foaming. You can drill holes in the rock for zip ties to make them less noticeable. Use a finishing resin to coat the foam with sand, and protect the foam from UV. I bought Z-Poxy finishing resin for this.
Here is a link to the DIY thread entitled, "The Foam/rock Projects Here Thread":
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