I made one myself and it works great, now anyway. I suspected something was amiss with my car so I ended up with an odd problem. The applicable failure mode was the hysterisis, the fans came on at the right temperature but went off at a lower than expected temperature. The lower temperature was below the thermostat so once the fans came on they wouldn't go off unless the engine got real cool and the thermostat closed, but then it stopped water from going to the radiator which is where the fans were. I just increased the resistance to raise the lower temperature above the upper temperature of the thermostat.

Now if I can just figure out how to bypass the ignition switch to keep things from roasting after I shut the engine off.