I was watching some youtube skoolie videos a while back and there was one that had these insulated curtains. It was basically one of those sun protectors that go in the windshield to protect the seats from the hot sun. They said it really helped keep the heat in during the winter. I don't know if they just bought some of those windshield protector and then made them or if they are on the market already where they can be bought but that might be a solution to help keep you warm during the winter.