A second on the milk house heaters as a safe and effective way to temporarily fix the problem. We used one in a well pit one winter when the wooden well cover was busted and it dropped to below 0F for an extended period of time. Tied it off and lowered it in with an extension cord, after checking that the pit wasn’t going to go ‘boom’ due to being a particularly nasty example of an enclosed space. A note on that, a lot of well pits and well houses can be very dangerous spaces with a build up of hazardous gases at the bottom. If it is a big one, i.e. deeper than four ft, such as ours (14 ft deep and 8x8), don’t just pop a ladder in and go to the bottom, you may not be coming back out.
But, I’d also suggest getting a well digger out, not a plumber, to have a look. That really shouldn’t be happening at such moderate temperatures especially if the structure has an enclosure, and any electrical heating element is going to rapidly be more expensive than their visit… Something may not be draining correctly?