I use a 'work light' grade bulb which is just a little more robust and covered with silicone or some other heavy duty coating. The plain bulbs just blew out when knocking around inside the chamber. I didn't want my power cable resting on the floor in case there was a leak or moisture, so it hangs and the silicone covered bulb protects better.
A light bulb couldn't be more effective as it works faster than the freezer itself. I tried a couple other things and they were way overkill.
(of course, I brew in stainless, so the light thing isn't an issue.)