Darin,
I am (slowly) putting together a 54 gallon bowfront corner tank and a foam wall background on the two straight walls will be part of it. I have cut the egg crate for both pieces so they fit snugly between the bottom of the tank and the top lip. then I got a piece of 1-1/2" PVC and varius bends, put them together and then brushed epoxy on the immediate inside of them. After it dried, I sprinkled them with reef sand. Then I laid them on the egg crate and foamed it with "Great Foam". no rock, just foam. I let the foam dry for a couple days then I painted on epozy and sprinkled with reef sand. I am only half way done putting the sand on, and then there will be a touch up layer also. It is relatively inexpensive and kind of fun to tinker with. My other wall will be trickier because I want to hide my overflow box and the return line from the sump.
just as a guide I used:
1 sheet of egg crate from Lowes gave me my two pieces, approx 21"x24" (about $11)
4 various PVC bends and a short length of PVC (about $10)
two cans of great foam for one wall (got them on sale for $2.99 each)
two part Epoxy, might barely have enough for 1 wall (about $10)
I think I am going to be very happy with the way it's going to turn out. I may even make four walls, in case they need cleaned, pop two dirty ones out and pop the two clean ones in.
if you decide to do it, let me know how it goes for you.