Jumping in, as well, with a question: How exactly will the egg crate help you build up a hill? Mound up a hill, then embed the crate into the surface to stabilize? Stack crates cut to size like contour lines on a map and then add the gravel?
And, if the egg crate is embedded into the gravel, what would prevent the white plastic from becoming exposed if the gravel flatten outs?
As for finding the egg crate panels, Navigarden, look in the lighting section at Lowes and Home Depot. They are used as "light diffusion" panels for recessed flurorescent lighting fixtures and are plastic 2 x 4 foot panels, normally silver or white colored.