A small (8 by 14 foot) shed is setup for field work at a remote site. The shed can be accessed by truck as it is next to a dirt road. In this shed is just a shelf bed and some camping basics. It is a 3-season shed and is poorly insulated, but it is dry and solid.
For field work on the edges of "3-season" weather, when it is very cold in early spring or late autumn, how could this shed safely be warmed? Expected daily temperatures would range from 40-60F with extremes as high as 80F and as low as 26F (below freezing). Field work is mostly manual labor but could require some dexterity, and field visits would range in duration from 3 to 10 days.
Some options I am considering: There is no wood stove in place but that is an option, though given the shed size it would need to be a very small stove. There will be a 100Ah 12V deep cycle battery available, but at least 20Ah of that will be used for other purposes and given electric heating uses a lot of power I am not sure that is a good option. Camp stoves seem like a dangerous option, but what about smaller and lower-tech options such as the small dishes of high-proof alcohol which are burned for cooking?
Some additional context: This shed is already equipped with a battery powered smoke and CO detector along with a fire extinguisher. A propane cooking stove is used in the shed with windows and doors open for ventilation. Warm, dry clothes, adequate sleeping bags, and plenty of food and exercise are part of these work visits, so I am not worried about hypothermia, but I am asking about additional warming options for comfort and morale. A 4wd vehicle can pull up pretty close to the shed, making propane or a wood stove install an option.