Maybe you could take one of those bun warmers that some of the nicer cars come with, for heating the seats and just wrap it around the bottom of the pan on the transmission.
insted of jerryrigging something, go to an auto parts store, tell them you want a 100 watt stick on pad engine heater, and give them the $15-20 that they want for it, clean off a spot on your tranny, and you are done!