I used one of these for fan speed control, gives full control from very slow to full voltage.
https://www.amazon.ca/gp/product/B07RQKS4PX/
and I haven't installed it but plan to use a simple cable actuated heater valve on the heater hose, primarily so I can shut the heat off in summer, I expect fan will give enough control for winter:
https://www.amazon.ca/gp/product/B001B7BUAU
Make sure your engine does not require constant flow from the heater.
If you want a thermostat to set temp in the cab, here is an idea I have not tried myself:
There are various 12v coolant valves, the ones with 2 wires are simple on/off valves. You could control it with something like this:
https://www.amazon.ca/Intelligent-Pr.../dp/B07D28NFRS