There are ceramic heat emitters that screw into a regular light socket but only give off heat. If your house really does get that cold, I think one of those would be the way to go.
One thing I don't know: should they be put at the top near the basking bulb or further down the cage? My feeling is that as the ground retains heat longer at night, their instinct during a cold night would be to move lower, but I really don't know.