The snow/armpit thing would only work with thermopiles you engineered yourself, mind you, because you could make the physical separation between the hot and cold junctions as large as you wanted. The pre-made thermoelectric generators have the hot and cold side pretty close to each other. I have read there's a danger of the heat from the hot side overwhelming the cold side in some applications (i.e. where the cold side isn't actively cooled). That shouldn't be a problem in your scenario, but you'll have to do something like set the TEG on the cold ground and sit or lay on the hot side. Same principle, though: use the cold to your advantage.