When I had a waterboxer (now have a EV) I gought a nozzel band heater from
McMaster -Carr. It was only 30 watts or so but did add alot of heat. It is like
a big hose clamp and I mounted it on the big coolant inlet pipe on the left side
of the engine.

i think it cost less then $20. While it would never get the engine "hot" it
would get it at a higher temp then with out it and did help starting at -10f.