i used the pink Styrofoam and cut it into rock like shapes, progressively smaller and stacked them on top each other using silicone and then i painted it with cement, first mix it runny and put a thin coat on, helps if u rough up the Styrofoam and bit with sand paper so it can grab on, then a thicker coat, another think coat and then another thick coat.
just make sure you get Styrofoam that dose not have the fire resistant or mold resistant chemicals on it, i cut mine into 3 pannels and make sure you do LOTS of dry fitting, i ended up putting a rim a 1" styrofoam around the boarder of each panel and then silicone that too the tank, i had space for the filter intakes and out takes and heaters..power heads were hard to fit in though..
any questions just ask