I had the same need for a heater and was about to install a heater in an
insulated cabinet in my cold basement, until I found another method
using a large enough container that will hold your carboy, but keep it
surrounded by water. Use an aquarium heater to keep the water (and thus
the fermenter) at the desired temp. I added a small aquarium pump I had
handy so the water would be stirred around and more evenly heated. Note
that you may want to set the heater in some water without your fermenter
a few days or more beforehand so you can get the temp adjusted to
exactly where you want it. Someone else also made a good point of
putting a "rain loop" in your electrical cord, so if you have blowoff,
etc. wort or water will not travel down the cord to the electrical
outlet. Also consider wrapping the water container with an old blanket
or somthing to insulate it if it's really cold and you loose heat to
fast. Hope this helps!