Call someone in your area with a well and find out how they keep the
well house from freezing. Whatever method that is will probably be
your best bet because they are looking for the same thing that you are
and experience has given them the best solution.

Well, if they didn't have a death wish they purchase a pump house heater.
They come in 120 or 240 volt versions and have a thermostat that cuts in
just above freezing.