what kind of cheap thermostat could I use to control a passive solar water heater for my hot tub?

Image by kahunapulej
This is how we heat our hot water on sunny days. On days that are too
cloudy, we burn wood in our fireplace to heat the water. Fortunately,
there is usually (but not always) enough sun each day to heat our
water… as long as you aren’t the last one to take a shower, that day.

Question by flagnsnarf: what kind of cheap thermostat could I use to control a passive solar water heater for my hot tub?
I want to build a batch heater or a passive coil system on a painted black corrugated metal backing. But I want to be able to stop the flow when the water gets to a set temp. It would be great to be able to just put in a car radiator thermostat but it would allow the water to get too hot.

Feel free to answer in the comment section below