Many people use the two way controllers, STC-1000. They are fairly easy to use and setup, and only cost about $20 from Amazon. There are multiple threads about them and how to wire it. A heating pad, lightbulb in a paint can, a cheap space heater, and more have been used for heating. If you don't have a way to control it though, you're going to have your freezer fighting with your heater, and that's a no win.