just my opinion:
Place your rocks on the glass bottom FIRST and place sand into aquarium after rock is in place. I prefer a very shallow sandbed (less than half inch depth) or none at all.
Choosing to use eggcrate or a deep sandbed can make your reefkeeping experience less than optimal- to say the least.
These are important choices that will definitely impact the ease and success of your reef aquarium.
I now run a reef aquarium that's half barebottom / half shallow sandbed and that's my choice after many years of experimentation with many types of substrate.