Thats right about the eggcrate. I have held 300lbs up on eggcrate with just cut pvc rings under it. It is stronger than you think, plus, my structure is crossbarced in several places. It will hold the weight with no problems.
I am going for two areas of rock with a central canyon, i want the rock to slope, but also be relatively flat on top. With 4 feet from front to back, I have plenty of space to achieve this.
P.