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.