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.
Tattooist: What do you want in the dream catcher?
Client: Do what ever you want.
Tattooist: Can I put Cthulhu in it?
Tattooist: Then don't tell me to do whatever I want.