"Egg crates" are actually light diffusers used for flourescent lighting and can be found in many hardware stores. They should be more than enough for your sized tank since you only need to go 20 inch wide by 26 inch high, just need to be trimmed down (I use pliers and clip away.)
You can also look for vinyl coated hardware cloth (I just learned about this on the simplydiscus forums).
As for keeping them in place, you can use suction cups (like for your heaters or you can buy some made for dividers - someone on this website even suggested using ones he found in a hardware store to hold shower curtains or something like that) on each side to hold them in place or silicone it in place (which means you got to drain the tank and let it cure).