@cwk266,
Not knowing where the temp needs monitoring and that you said a thermostat is accurate enough I would suggest you use your thermostat.
Use a cooling thermostat as the contacts will close when the Temp rises.
Run 120 volts through the Stat to energize the off circuit of a two pole mechanically-held 20 amp contactor. Install a momentary pushbutton to reset the contactor to the closed position manually.