I'll second the Nalgene bottle idea. Since I already have the stove, the bottle and the water all it takes is a little extra fuel. Before I doze off I'll lie on my back and place the bottle between my legs at mid thigh. That warms up the big femoral arteries and spreads the warmth all over my body. Ahhhhh.........it keeps me 79% warmer.

The bottle stays warm well into the night.

This system carries no fire or CO risk. Just remember to keep the lid tight.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.