I have one of those small Mr. Heaters, sits on top of those green propane bottles. I usually bring it with me in either my tree stand or my portable bind, when the temps are very low. I use it to keep me warm and extend my hunt, but only use it after I start to shiver. Then I turn the hearter on, warm up, and off it goes. I can do this several times untill darkness comes or I leave. Works great, smaller than a sleeping bag and doesn't cost much.

T