Radiator heater and a plug-in thermostat like this http://www.amazon.com/Lux-Heating-Cooling-Programmable-Thermostat/dp/B000E7NYY8/ref=sr_1_9?ie=UTF8&qid=1401635080&sr=8-9&keywords=radiator+heater
Can't you add more lights? To me it seems a waste to warm the plants with something other than more lights. I have my lights on in the night and off in the day so my temperature doesn't fluctuate much