[MatPat]

Styrofoam works in the same manner as egg crate and may be a bit easier to cut depending on your skill level and available tools. The plus with styrofoam is that you don't have to worry about gravel or sand getting under it and creating a pressure point like you do with egg crate. Styrofoam is cheaper too

 

[MatPat]

Eggcrate is usually the stuff that goes over fluorescent light fixtures in commercial applications. It is used as a light diffuser and probably to protect the bulbs somewhat. I have seen it used in residential applications as well but not very often.