Egg crates are found at hardware store under lighting or ceiling panels. Any of the big box store should have it. <eg HD, Lowes.>
You put the eggcrate down on the glass, then the rocks on top of that. It spread out the weight of the rocks & prevent sharp edges from scoring the glass. If your rocks are not stable, you might want to silicone them rather than just simple stacking ... or they might fall over & damage something.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.