A small, sealed igloo, with a cold air sump raises temps by at least 50F degrees, with just your body heat. Too bad you fools dont know the facts of reality. Given all the clothes you're allowed to take, the debris between those layers of clothing, between the clothing and the poncho and leggings, and between the poncho and the 10x10 tarp that you wrap around you and tie. That's how you dont need the sleeping bag, Insulation is insulation. It doesn't matter in what form it comes, as long as you dont have to stow it in a pack and hike with it. All insulation has to do is trap air around you, holding in your body heat.

with cammies, balaclava, glove liners, with booties, mittens and hood made out of "breathable" mylar, laying ON a sleeping bag, and pad, at 20F, in 20mph winds, a net hammock around my torso, a bugnet bag around each leg, under my cammies, with 3 drum liners wrapped around me, without any debris between the layers, I have slept outside, no fire involved at all, no hot rocks, nothing, inside of a couple of SOL mylar bivvies. The inside one was made of 1.5 of their "breathable" Escape lite" bivvies, the outside one was their 2 person 'non-breathable" two person Emergency bivvy. I used no debris between the bivvies, I tented both bivvies over a ridgeline. All edges were sealed with tape. The 'trapped air" layer is THAT efficient. If I put debris between the bivvies and the layers of wraps around my body, I guarantee that I could sleep ok at 10F, With the MUCH heavier and many more layers of clothing that they are allowed to take, a double layered tarp shelter, with debris between the layers, a cold air sinkhole, a raised bed, is guaranteed to be good enough to let you sleep well at 0F, without a fire and without a sleeping bag.