I have described a solution an overheating problem with my IC2200H at Cooling an IC2200H.
I reviewed one of these things at Review of inexpensive Chinese thermostat – MH-1210. It had its issues, but as modified for 12VDC operation, I tested it for control of the fan in the above pic.
Nevertheless, the test demonstrated that the approach performs quite well. The thermostat was configured for cooling, setpoint=50° and differential=5° (to minimise the nuisance of short run cycles).
The above NTC 10k thermistor was wedged in between heat sink fins at the hottest part of the heatsink using a piece of closed cell foam which kept the thermistor in contact with the heatsink and protected it from direct cooling by the fan. For more permanent installations, I use thermal adhesive (sold on eBay as “heat sink plaster”.
The solution works a treat, the thermostats are inexpensive and give a direct reading of heat sink temperature as a bonus. The difficult part is finding a 12VDC one on eBay as most sellers do not describe them accurately (I have a replacement being shipped as I write, a result of the seller shipping a 230VAC model).