@Tayler_mann is probably referring to a Webasto heater. They run on diesel and keep the engine coolant at what ever temperature you set it for. I know people who use them, but it only keeps the engine warm to reduce wear on startup and reduce idle time for warm ups. I don't see a benefit to this situation.
I know at a cabin we had when I was a kid, we had an electric well pump away from the structure. To keep it from freezing, my dad built a plywood box lined with rigid foam insulation and we left a 75 watt incandescent bulb on throughout winter. I was amazed as a kid, how warm it would be in there when I would go check it. As odd as it may sound, I wonder if a somewhat similar idea may work for you.
A nicer heavily insulated cooler should be enough of a barrier. If you were to use a microwaveable heating pad (the kind for muscle aches - ours have corn or beams on them) and nuke it for a minute or two when you get home, then toss it in the cooler. That combination should be more than enough to keep the contents of said cooler warm.
On milder nights, the cooler alone would be enough. I almost think that some form of heating medium, aforementioned heating pad or even a bottle of water, on the floor near your heating ducts, could get hot enough to do the job as well.
