Stove/fire/ outside, boil water, hot water bottle inside

Heck, you could even carry a few bricks/tiles and heat those instead of water, outside of course.
Carbon monoxide poisoning from Sterno. [Can Med Assoc J. 1978] - PubMed ResultA high school student became ill and later unconscious while working over
a heating table set over three cans of burning Sterno. Measurements of
1000 to 3000 parts per million of carbon monoxide were obtained around and
above the apparatus. Although the room was well ventilated there was
incomplete combustion of the canned heating fuel because the apparatus was
surrounded by aluminum foil, which resulted in poor oxygenation of the
flame area. This case demonstrates the hazards of carbon monoxide
poisoning from incompletely burned Sterno.
You've replicated this exact scenario.
All that is required is that you supply a little too much fuel, just a slight twist of the knob, and you're making so much CO, two breaths and goodbye
The space is just too small for open flame like that