Heat rises, so the greatest heat loss would be through the roof. I wonder if some flexible closed-cell foam could be added around the ceiling? Something with some ability to be curved, yet stiff enough to hold itself up against the ceiling, would help a lot. And putting some insulating material on top of the roof (provided it isn't heavy and doesn't accumulate snow) would be another way to hold in the rising heat.
Secondarily, any drafty cracks or openings would lose heat, but eggs are pretty good in that regard. The next thing that comes to mind is some shrink-plastic overlapping the windows
on the inside, which helps a little bit... or you could even put some pieces of Reflectix against the windows and vent, at least at night.