I use a DeLonghi brand oil-filled electric heater with a thermostat that goes down to the 30's. Heaters like this are widely available and not very expensive (mine was $50). You could also plug an electric heater without a thermostat into a separate thermostat like the one in the link below:

Here is a link that might be useful: Durostat

I was thinking about an oil-filled unit, but I don't think it's enought to keep the GH above freezing when the temps get into the teens. I have a 20 amp service in there so I can only plug in one unit.

Does anyone have experience with the Pro-com units (propane or natural gas) you can get at harbor freight (or other stores) Specifically how low can the temperature be set?