I have taken to hunting out of my chair blind. It is the easiest/safest to fall asleep in
Since it is relatively small and enclosed I have generally just closed it up as much as possible with half length foam sleeping pad on the ground to insulate then open 3-4 chem warmers and drop them. The foam pad also eliminates leaf rustling. I bought a big box a while back and just ran out so I was thinking of trying something new. Bought a zippo and I am going to give it a try.
I also don't like the fact that the chem warmers are one use and throw away. If I hunt 20 days in the winter that is at 80 of them I am tossing in a landfill.