Electric fan thermostat

I have an electric fan now (the cheap 11" Tripac fan, fits perfectly)
and would like to find a thermostat that screws into the existing
fitting on the downflow rad. (thread size 5/8-18) 

Has anyone found one that fits and has a turn-on temp that's about right?
Any other solutions?

Thanks, Ulix

P.S. my car runs and drives, but some issues need to be worked out,
report later