Using a cinder block, I ran a 60W light bulb into one of its chambers and then sat the water dispenser on top. The heat from the light bulb is concentrated just enough to keep the weather above freezing. It's worked perfectly all winter and hasn't been a blip on the electric bill.
One trick to make the cinder block easier is to chisel away a small slot on the bottom of the block so the extension cord slides in more easily. Also, a note about safety. Make sure the cinder block sits up high and doesn't get caught up in any hay or pine chips. Fire is a real concern with this setup.