Hot wire foam cutter
There are several DIY ways to construct a cheap effective hot wire cutter.

I bought a couple of brass tubes at the hobby store and used a piece of 1"x2.5"x 30" wood to create the handle in between. I got a couple of springs and some plastic coated picture hanging cable to create the tension line across the top. The reason for this is to keep tension on the hot wire when it heats up and gets longer.

I used NiChrome wire that you can get at the hobby store for the hot wire. At first I tried an old train transformer but it didn't have the juice to get the wire hot enough for a large block of foam. Then I hooked up my car battery charger and that seemed to be just the right amount of power to heat the wire which was about 30" long.