Hi
I did the same as Bob, £20 for a thermostat, connect it to a brewbelt put the built in temp probe on the outside of the FV with some bubble wrap over it, set the desired temp and they just work together.
I now have a temp controller that I can plug a brewbelt in the heat side of things and a large fan in the cool side of things set temp to 12 degrees and when it goes over 12degs the fan comes on, if it goes under the brewbelt comes on, I will struggle to achieve 12 degrees in the summer but I will be aiming to keep it around 14 - 16 degrees then.
Oh I brew lager by the way, best brewed at 12 degs with proper lager yeast.