Hey andy, I used portland cement, number 3 i believe, but you can use most brands as long as quickrite is not used. The key is the curing, after you got it done, soak it in salt water for a few weeks. just dump a few cups of salt in the water, change water a few times. if you dont cure it, the ph may go through the roof and kill your fish.
Im also hoping the styrofoam will act as an insulator, like the old beer coolers, and my heater wont have to work as hard.
if your concerned about ph, use your bondo and put a layer over the background, that would totally seel it up. and bondo is cheap and easy to use, 10 bucks a pint or 27 bucks a gallon, available in most hardware automotive sections "bondo fiberglass resin" great stuff