"If I didn't have so many damned cats, I would try a yurt. I think they could bust out of that pretty easily though."
I spent a weekend in one of the insulated yurts in an Oregon campground. Pretty solid construction, small electric heater, couldn't see any gaps, no drafts, framed door and windows, solid door.
If you happen to find yourself along the Oregon coast, find one of the State parks that has them for rent. Quite nice, actually, even in a cold, windy, rainy October.
Pacific Yurts Sue