.
A round 2" hole in the floor with the vent window opened a 1/8" should handle
the ventilation with no problem along with minimal heat loss. It requires no power and is self regulating.
You could try raising the bed height towards the warmer air near the ceiling.
If supplementary heat is still required, consider using a 12v electric blanket. Most use 4 amps or less and are relatively cheap ($25-$50).
However, if it were me and I was cold. I would just get a belly warmer.
Maybe two, if it's really cold...
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