GotMarko writes in with a hot wire foam cutter made from some posts on MAKE/Instructables – “A hot wire foam cutter and the jig I used to cut 4-inch styrofoam balls in half…Instead of the CD case project box, I just used a three wide plastic outlet box from HD to hold the dimmer and transformer. I cut the tabs off the sides since they were just in the way. Since I had some 1-by material around, I used that to make a small bow for holding the cutting wire. Twine around the top is used to tension the wire and can be adjusted as the wire heats and lengthens.” – Link.
Hot wire foam cutter – Link.