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.
Primary: Whiskey Warmer
Secondary: Berliner Weisse
On tap: BBBBB (Big Batch Barrel Belgian), Richardson Pride Special Bitter, Orange Honey Habanero Wheat, Men are from Marzen, Barrel aged Porter, Pumpkin