I use a Coleman Propane portable heater that looks something like the one in the first pic here, but it has a sturdy base like the second pic. I make sure the cabin is ventilated and shut it off when I go to sleep. When I was at Avalon last week it was about 46 degrees outside and a toasty 75 degrees inside. Mine also has a handle like a lantern. I have had this for years. My Father used it on his Bayfield 25' in Alaska as well. Thanks Dad!
Sorry Sven, I'll be hanging out for the Harbor Christmas Parade.