Survival Blanket
There are now several different types of survival blanket on the market. The most common one you have probably seen is the one marathon runners get wrapped in at the end of the race on TV.
These blankets are designed to reflect heat and they do this very well. They are designed to be used over clothing for max effect. Considering how small, light and inexpensive they are they are well worth carrying if you are gaming or hiking anywhere away from shelter. I have also found if you put one inside your bivi bag over your sleeping bag it makes the bag warmer but be careful doing this as space blankets are not “breathable” and can cause condensation to build.