Check out "Zippo hand warmer".

It uses the same fuel as a zippo lighter but it doesn't have a flame...it has an element that glows red. Then that is covered by metal with holes in it...and the whole thing goes into a leather pouch (so you don't set yourself on fire).

Seems to me one of these hand warmers would go a long LONG way in your sleeping bag...or under the covers...as long as your head was out so you weren't breathing the fumes.

- MC