Thinking of other ways to keep the canister heated - what about a mug warmer? A mains one can be found here http/cgi.ebay.co.uk/Simpsons-Mains-Operated-Electric-Mug-Warmer_W0QQitemZ360243231709QQcmdZViewItem?rvr_id=&rvr_id=&cguid=2d02127b1260a0e202a4d0b4fffa1690. or you could get a usb one plus a mains/usb adapter. What would an ideal temperature be? Some of these mug warmers claim to keep the temperature at 40-60 degrees.
You don't want it too hot, or you'll kill the yeast, or cause it too consume the sugar mix too rapidly. Unless your surrounding temperature is extremely cold, room temperature is going to be fine for fermentation CO2. At least, that has been my experience. During the winter, my home temperature plunged to around 18 degrees celcius. The fermentation still worked fine. Remember, usually the canisters will be near a heat source already (lighting, electric outlets, etc) and the tank itself generates a small amount of heat. This should be enough. I wouldn't worry about it much.