Northern Tier the national boy scout base in Ely, MN has used this method for many years. They have each scout wear a 2 cup Nalgene round there neck in a nylon cover and strap. Since keeping hydrated is important the standard question is how many Nalgenes have you had today, show me your bottle. At night the bottle is filled with boiling water placed in the carrier with a fruit bar. Instructions are if you feel cold wake up take a bit of the bar and a sip of water this will get your metabolism going to help you sleep. The warmth of the bottle helps until the body heat warms up the cold bag. Use a Nalegne nothing less. I used a flask one year in a snow cave got up to water the snow and when I got back in I stepped on the bottle. A couple of hours later I could not sleep and didn't figure it out till morning that the bottle cap was off and the bottle empty. It was -25F and I using a down bag. Never a problem with the Nalegene.