A fridge normally chills the freezer section on top and the bottom gets cold as a consequence. If you are just doing ferments, you won't be anywhere near freezing.
Just turn the control in the fridge all the way cold and the controller will switch it on and off when needed. Since the fridge thermostat is set colder, it will always kick on.
Alternatively, you could bypass the thermostat completely, but most people want to keep it intact in case the fridge is used as a fridge again.