Foreword: You need to be very careful about cutting through the walls of the freezer as most of the walls contain the cooling element. If you puncture a cooling element, all the coolant will leak out and you'll be left with a fancy box. In my freezer, the was an area in the bottom right side that contained the compressor, enough room to install the controller, and a thin wall obviously devoid of any cooling parts.
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.