Winter boondock... Cold cab over. Need ideas.

I used a $5 closed cell foam pad like backpackers sleep on, cut to fit. In my rig i can even squish it into a space between glass and trim, so it stays put.

Another idea: It's ugly, but you would be amazed how much added warmth you get from a large tarp, sheet of fabric, or anything similar, thrown over the room if the cabover and tucked in at the bottom. Gives you several inches of trapped air insulation all around.