I would try to avoid damaging the insulation to add an additional sensor, and if you just try to sneak a cable through the front door, the added small gap will

I would try to avoid damaging the insulation to add an additional sensor, and if you just try to sneak a cable through the front door, the added small gap will let in enough moist air to give you a problem with ice forming. What kind of controller does your freezer use? You could try to tap into the analog singnal inside of the controller or try to find a serial port (most temp.controllers have some kind of protocol specified where you can adjust timing parameters (P.I.D. constants, duty-cycle limits, ...)). Even a 7-segment display can be converted back to digits on a serial port easily.