Brew Belt Fermenter Heater, 15 Watt (Brewer's Best)

The Brewer's Best® / Vintner's Best® Brew Belt Fermenter Heater will help you maintain a constant fermenting temperature of 68-75° F for all of your fermenting beverages (beer wort, wine must). Environments in your home may not be ideal for keeping your fermenter at the perfect temperature, but the Brew Belt is here to help. It will maintain a constant temperature for up to 8 days. 15W. Comes with two spring sizes to fit most fermenters.

You can hook this up to a thermostat if you want to be even more precise.