Grow Box in unheated Greenhouse

I'm just about ready to harvest and I want to continue growing growing this winter. My CFL growbox is in an attached greenhouse which is unheated. I do not want to heat my entire greenhouse just my growbox. I was thinking of putting in a heatpad under my plants to keep the temps up when my lights on on the off cycle during the colder months. Will this work? How about an engine block heater or even a high wattage green light? Any suggestions are welcome.