I just put a heater (I use a brew belt, but a small lamp in a coffee tin would work as well) in the freezer. In the summer time, I connect the freezer to the temperature controller, and in the winter I connect the heater to the controller. In the spring and fall, where a day can be both well above and well below my set point, I leave the heater running constantly while I hook the freezer up to the temperature controller. Not the most efficient way of doing things (that would probably require the use of a dual-stage controller), but it works out rather well.