For heat, I use a ceramic pet heater like this. Pet heater. I think it might be 150 watts. Also added a small fan blowing on the heater so the fan and heater turn on at the same time. This is in a 2 level insulated box with room for 8 carboys on each level,though sadly it has never been that full. Heh. It seems to work quite well.
It also has 2 rancos on the outside of the box that can get quite cold in the winter. -10F.? Just guessing there but probably pretty close. I think you'll be good there.