I think there are a few different ways that you can go about this.
I went for the budget version - a cheap thermostat from China via ebay. I plug it into the power socket and than plug the fridge into the thermostat. A temperature sensor goes in past the door seal.
I'ts not really accurate. I have it set on 15C to get a temperature of about 13C and there is a +/-1C variation on top of this. Then again, for around $20 Aust. what can you expect. These thermostats, by the way, are aimed at reptile fanciers for their terrariums.
Others may have more elegant and accurate solutions.