I have made a makeshift sauna with hot rocks and a tarp. find a hole or dig one(not at BWCA campsite) have tarp touch ground and hold down edges with rocks. Close opening, heat rocks in fire, and bring them in 1 by one with hot pad or something similar when you have a few in there pour a bit of water on. Tarp doesn't hold the heat long so don't open it, and have some back up rocks ready to go.