Good ideas in this thread... but I have always been happy with using a big old Sears auto battery charger and .015 or .012 7 strand stainless control line wire. I just use little alagator clips to get the power to the bow and move them in or out to control the wire temp. I like a simple lightweight bow so that I can use to cut without a helper. A heavy bow makes that much more difficult. Doing that I have cut hundreds of foam wings for CL combat and stunt locally. Did a video on the process some years back that my son is still selling on the web if anyone is interested just search for foam wing cutting on Google and follow the paid links. It is full of KISS principle foam cutting advice.
Have fun and do good things!
Bob Furr