I built an insulated closet to house a 2nd refrigerator in my pavilion. I heat it with a 250 watt halogen worklight. I connected a line voltage thermostat to a receptacle and keep it set to 60*. The refrigerator wouldn't run when the outside temp was under 40* so the freezer would thaw. The heat also keeps the humidity from condensing on the metal refrigerator case.
You just need to keep the compressor and oil warm enough to operate. That can be accomplished with a 40 - 60 watt bulb sitting near the compressor.
