It would be cheaper & safer if you put some cardboard boxes in there, lying on their side, with some old blankets in it. They'll curl up inside & their body heat will keep them warm.
You could even get some little beds at petstores that have a heat-reflective layer inside. The cat will feel warmer, but there's no electricity to worry about. If you can get their trust at some point & get some help to have them spayed/neutered, that would be great. Good luck.