I have a little battery operated clam light that has a fan in the middle, it pushes just enough air around to help. It has 4 double A's and last just about all season, my shelter is small though, and I am fat so I don't get real cold, real fast
I have the heater that points up I have used it in my clam six pack and my clam tc x4 it is a friggin roaster but when I bring my little bro along he likes it as does my mom, when it running I fish outside no matter what the temp is