So here's what I did to assemble the rock wall:
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Cut the egg crate to desired size.
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Arrange rock and PVC to desired positions
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Zip tie rock and PVC to the egg crate.
Squirt the foam into place. I used the tetra pond foam sealant.
While the foam is setting I sprinkled rubble rock on the foam.
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Once I get the whole thing foamed and the foam cures I will "paint the foam" with epoxy and sprinkle it with dry sand.