"Does anyone know where I might find float switches that will complete the circuit when the coolant level falls?"

Randy, in 20 minutes I found the four float switches I'd squirreled away in the garage, unused, untried. They are the same as in this current ebay listing:

https://www.ebay.com/itm/142763222556

They mount horizontally from the side and depending on which way you orient the float, they operate closed circuit or open circuit with low liquid level.

Could be you've seen these and dismissed the idea of putting a large hole in the side of your reservoir to mount them. That may even be the reason I left them linger unused.
--
Art Benstein near Baltimore

From Rock Auto's December 2021 Newsletter:

I was in college in Colorado in 1976, and my wife and I bought a Mazda 626. Our homes were in NJ, and I decided to add an aftermarket cruise control to the car to help make the long drives back and forth more comfortable. I put in the control unit, solenoid and bellows for the throttle, brake switch, and the pick-up and magnet on the driveshaft. The magnet had to be tapped in position so it would not fly off the shaft at speed. Several weeks after adding the system, a horrible noise developed at 55+ mph. When the car slowed, the noise went away.

It had to be a problem with the transmission. I took it out, tore it down in my apartment, used all the same gaskets to rebuild it and stuffed it back in. The horrible noise still occurred. I was frustrated. My wife (who knows everything) said it was something I did with the cruise control. She was right. I wrapped the tape backwards on the driveshaft and over 55 mph it flew out and hit the exhaust. Under 55 mph centrifugal force was lower, the tape fell back in and the noise was gone. I re-wrapped it in the opposite direction and all was fixed...

Often times people that "know nothing" take a simple approach and figure it out with common sense!

Bill in New Jersey