Which leads me to...
hand-warmers powered by Zippo lighter fuel...
I know that they used to sell these things years ago. You can still get them used off of eBay, but not brand-new any more. Why? Did some people burst into flames using those things? If they are safe, I was considering grabbing one or two.
The still make them.
Cabela's sells them here They work great. I use one duck hunting.
They are safe, but you obviously need some ventilation since it is "burning".