I use a brewbelt, 15W. So far it keeps a full sized upright freezer warm enough for fermenting two 5 gallon lagers in a detached garage. (50F inside, 28 - 33F average outside).
Using a fan to circulate the air is important. What you would want to do is mount it to a wall and use a fan to circulate the warmed air. I have a desk fan in my freezer right now but I'm working on making a computer fan panel. It's similiar to a stir plate build but without a rheostat or magnet.
My process for cloning a beer
1. Put on lab coat.
2. Drink beer and take notes.
3. Repeat Step 2 until I have a recipe or pass out.
On deck: Barleywine, Dampfbier, Belgain Golden Ale, Hazelnut Sweet Stout, IRA, Rye-weizen, Wit, Uinta Hazel Clone, Uinta Yard Sale Clone.