I'm also in the process of building a chamber and will incorporate some sort of heating method. Seems like a lot of people use a simple incandescent light bulb (60 - 100 W?) with a shield. I'll either do the light-bulb method or buy a cheap space heater.
I am using the Williams Brewing Heating Pad made for fermentation chambers. It is pretty nice, has adhesive on it to attach to whatever surface you want, and heats well. I have it hooked up in my ferm fridge on a two stage controller, when I first plugged it in the fridge was at 36 and it took it up to 68 in less than an hour, with a good amount of liquid in the fridge.
There have been a few posts of melted fridges on here, but I think that's usually an exposed bulb up against the siding. If it's in the can, and suspended (not touching the sides), and your temp controller works, it should shut off before it gets too hot. But I don't use a bulb anymore.
Your local pet store should have infrared or ceramic bulbs in the reptile section. They screw into a normal lamp or socket and are specifically designed to heat enclosed spaces. The IR light also won't hurt your beer!