I read through the entire thread and I'm impressed.

I saw Jone-E handwarmers mentioned as a heat source and I concur that they can be troublesome. I had the same thing happen to me -- working great until you put them in the sack, then they go out.

Believe it or not, Zippo has a handwarmer similar to the Jone-E, but they seem to actually work. That is, they get too hot burning in the open and moderate somewhat in their little supplied sack.

Of course, these things burn fuel and make CO (I imagine), so you don't want one in a bivvy if you pull your head inside. My experiments show they work well if you leave your face exposed outside the bivvy. (Exhaled moisture from your breath doesn't help your insulation stay dry, anyway...)

These things might be an option to an external fire, especially in areas where they are hard to build due to 150" of annual rain.

.30-06 Springfield: 100 yrs + and still going strong