See attached drawing. We placed the sensor 2/3 hight at the back of the fridgebox.
Thermostat contact is in parallel with the original mechanic thermostat. So you van use both (be sure to turn off the mechanical thermostat when running on electronic thermostat).
Thermostat setpoints: switch ON at 12,5 Celsius, and switch OFF at 9 Celsius.
The bottom temperature of the fridge box will be much lower (5 Celsius).