The Country Squire has about 200 cubic feet of space in the cabin. The walls and ceiling are insulated with Styrofoam at about an inch and a half thick. We bought a little electric heater that measures about 8x6 inches and tried to use it once while in the 30-40 degree range. It was overkill! We have found that just two warm bodies in such a close space keeps it nice for sleeping. We keep the vent open of course to breath but also to keep the humidity down. Just the act of breathing puts off a bunch of moisture. We keep the little heater in the camper and would use it to pre-heat if necessary but that has rarely been the case. For us cold is more bothersome during the day (awake) hours than at bedtime.
Other options:
Heat up a brick in or by the fire, wrap it in a towel and put in with you as you tuck in for the night.
Line your floor with some Heat Tape used for keeping pipes above freezing. Some of that may even have a thermostat built in.
Electric blanket
Run an in-floor manifold system that can be hooked up to a vessel heated by your campfire. Sounds a bit complex but if you camp as much in cold temps as hot perhaps its worth the engineering.
As always, be careful no matter what you do. There are many folks that are stone cold today just because they wanted to be a little warmer. There are many ways a man can go that lead to a shortened life.
