Egg crate under the sand is a big help. I have that to support my scape and there was no shift when my engineer went to town digging under the foundation.
Another option over the PVC idea is using plastic dowel rods. I believe you can get them from hobby stores. This way it is a smaller hole to be drilled. You could even secure egg crate and the rods together so you could almost do a linkin log sort of deal with your rocks if you ever want to redo the scape.
Don't you think using an egg crate in your substrate lessen the effectiveness of you filtration? Having enclosed the substrate with small partitions will prohibit any substrate living creatures from moving around to clean your detritus.
I placed my rock on the bottom of the tank then added my sand. That engineer goby can now dig wherever he wants!!