I purchased mine for $20 (Link removed due to seller jacking price up $100). eBay links don't last forever, but you can search for "digital temperature controller 110V." Many of them are designed to run on 220V, so make sure you get the proper voltage controller (110V in the US). This particular one allows for many customized settings include compressor delay, which is important as rapid on/off cycling can kill the compressor.
Dremel/rotary tool or jigsaw
Extra power cord
Step 1: Preparing the freezer
Measure the size hole you need to cut and trace it out on the wall. Double check you're not drilling through the thick part. Drill a hole in each corner and cut between them with either a dremel tool or jigsaw. If using a jigsaw, make sure the blade won't poke through far enough to hit and of the compressor lines. Make sure the controller will fit in the hole you cut.
An alternative to mounting the controller on the freeser is to use a plastic junction box and put the controller in the middle of the existing power cord. A blank faceplate provides a suitable surface to mount the controller. The finished results may not be as neat, but it gets the job done.
Step 2: Wiring in the controller
The way the temperature controller work is by cutting on and off the flow from the hot to either a heating or cooling element. Both circuits are normally open. When the temperature gets too hot, it will close the circuit and allow electricity to flow to the cooling element (the freezer). If I was using the freezer as a fermenter I might also have a heating element like a small ceramic heater or dehydrator connected that would be turned on when the temperature dropped too much.
To wire it up, the incoming (black cord in my case) hot and neutral get connected to the appropriate posts. Here the hot went to post 5 and the neutral to post 6, as shown on the diagram. The hot also needs to be connected to the post to switching portion before reaching the heating/cooling elements. Because post 1 and 5 both need to be connected to the incoming hot, you need to either pigtail a split in or do what I did an jumper a second short wire between the two. Next you need to connect the outgoing post shown going to the cooling element to the line that goes to the freezer (white cord here). Although the diagram shown this to be post 2, I found this to be a misprint and that post 3 actually is for the cooling element. Initially the freezer came on when the temperature was too low, which could have had disastrous implications if I didn't catch the mistake. Post 7 and 8 get connected to the included temperature probe, it doesn't matter which leg connects to which post. The neutral going to the freezer should get attached to post 6 along with the neutral coming from the outlet. The grounds don't actually connect to the controller and should be joined directly to each other.
Before sealing everything up, plug in the freezer to make sure it is all working as intended. To set the temperature, press set, use the up/down arrows to select the temp, then press reset. Test both when the probe it too hot or too cold. You can hold it against an ice cube if you need to cool it down.