if you use the lighter weight (1/2 inch) egg crate just as a base cushion, then fill to that level, then put the rock in, then add 2 more inches of sand, it should be enough. I think you'd be better off doing a full bottom liner just in case of vigorous tunneling. Last thing you'd want is that rockwork to topple. You might get away with a piece a couple inches larger than the rock..
I had a goby that rearranged a 15lb hunk of LR in less than a day. He wanted to live under it, and there was no stopping him...