Space blanket(s), P 38
While they are larger and heavier, military bivy bags are incredible; basically a waterproof shell for the sleeping bag system, they function as a “tent that you wear” and will trap heat much better than the “space blankets” (which I include with first aid kits instead of survival gear). In fact, you have to make sure your breath can escape, they work so well that if you trap your breath you’ll be soaked by the condensation.