Egg crate is, IMO, a waste of money.
I have one tank that has over 150 lbs of rock in it with a sand substrate and the rocks do not move and the glass does not crack or break.
I have stacked river rock and lace rock. I have a piece of Holey Rock that it over 40lbs. I have rocks stacked to the left of tanks and to the right of tanks. I have tanks that have high rock piles dead center of the tank. No tank has EVER cracked.
I have stacks of river rocks that are up to the top of the tank and have never siliconed them (what do you do when you want to take them out, rearrange the tank, clean the area under the rocks etc...?)