I have been useing a fish aquarium heater. (probe style)I have a bucket two inches bigger around than the fermentation bucket and carboys. The bucket has a racktwo inches from thebottom with the probe heater centered under it. The carboy sits on the rack over the heaterwith thewater from the bucket surounding it. The heater thermostat is a small wire thatcan pushed threw the bung rubber of the carboy. The temp can be viewed and adjusted froma digital display. A belt would be easier but were is the fun in easy.