I agree, flames in an enclosed space generates CO and other byproducts that will need to be filtered out and that air will need to be replaced by ambient air that needs to be filtered prior to allowing it into your enclosed space. So using MRE heaters (water-activated exothermic chemical heater) may be the way to go. No flame, no CO2 or CO generated, just heat. But those are single use items and you may need a bunch of them. Stock up now.