After spending a little time looking into options for a heater in the bivvy i have come across a few fairly good options, i will start by dismissing candles as anything more than a more comfortable light than LEDs  Making a small ammo box or paint can wood burner is popular for what canadians call hot tenting, it has to be well built using the correct sealants and chimney stack, A how to for making a stove without welding involved can be seen in the vid below. https://www.youtube.com/w