Has anyone tried using a heater in the top? I know I read back through and a lot of folks say you don't need it but most of those replies were a single person and could remove a panel to let heat come up. That's not always an option since my wife and I are both up there. We spent several nights in the 20's on our TAT trip last month, I think the lowest was 22 degrees and we didn't freeze but it wasn't necessarily comfortable all night either, especially when you got up to pee and let all the cold back in. We'd like to do some camping in the UP of Michigan this winter where it can dip below zero and want to figure out some sort of heat setup. Neither one of us can sleep in a fabric coffin... aka mummy bag.