We often use egg-crate for temporary barriers, but long term, large O. scyllarus will break through it. In fact, once they break off a piece, they well continue to work on it until there is a large hole.
There is a difference between the animal using a hard subrate to break shells and attempted digging. We often put a piece of plexiglass under a burrow to give the animals a hard surface to break on. A piece of flat rock also works. However, when animals dig and find an obstruction, they try to break it off. They do this in the field all the time when digging a burrow. Once they hit a bit of rubble, they try to dislodge it or break it off. You see the same thing in an aquarium. The animal starts to dig, hits the bottom, and tries to go through it. If they can't see through it, they usually don't try to break it, but no guarantee. I've lost most of my tanks when they dig in a bottom corner and chip the edge of the glass. I usually smear a healthy bead of silicone along the bottom edge and corner of new tanks and this stops the problem.
The major problem with a false bottom is that it provides the perfect place for some critters such as bristle worms that you dont want. I would just protect the edges with silicone and provide a small breaking surface and not worry about the animal breaking out the bottom - unless you are using glass less than 3/16".
