Your post is really hard to follow, so if you don't get the answer you're looking for, ask again.
I wouldn't use convoluted foam (egg crate as you call it) for anything. You don't need anything to fill up a 3" deep area, just make panels and cover the area with that. If you're concerned with noise, put some kind of a sound deadener like Dynamat in the area behind the panels.
No one lives forever, the trick is creating something that will.