How to heat

If you don't spray foam or something similar then your chances are real high that water will condense on the roof and drop on you. A wall mount " plaque heater" would work fine for something that size. Around $200 at farm supply stores \ northern tool. A 100 lb propane tank lasts well over a week depending on how high you run it.
Plus the don't take any electric and are vent free.I've used them for years