In my experiments, Quickcrete Acrylic fortifier used in place of water with Quickcrete FastSet Repair Mortar to be the ideal combination. I have used a few other Quickcrete cement products over the last year or so, but this one truly meets all the needs of the amphibian hobbyist.

The secret to carving foam is the right tools!

One of these:

Wonder Cutter PLUS! Styrofoam Cutter

and one of these:

The Wonder Cutter Styrofaom Cutter

and you can make ANYTHING out of polystyrene foams.

I also use a Dremel tool, a Sawzall, and all sorts of home made cutting blades. Then again as John can tell you, I tend to "nuke" just about everything I do. My methods get results, but they may not always be the most efficient. LOL