I use the little hand warmers. They throw out a decent heat. Place em in between thighs on femoral artery and you'll stay nice n warm.
Another one slung at bottom of sleeping bag keeps toes toasty warm too
I have to say... most of the Suffolk Group use Charcoal hand warmers or the Lighter fuel hand warmers.. supplied by me as I get them cheap lol : so we have two or three each

And it does work, we do have a regular that has a furry bunny water bottle as well

Make a flask of hot drink before you go to bed, if you need it its there ... in the morning you got a nice warm drink with out getting out the pit..
