I am working with a mutual aid org which has some resources at its disposal (money & labor) to help the folks who live there.
Of course, the people living there know best what they need, and with their current setups they mostly need wood for their stoves, or fuel for their generators, which are running electric heaters. But not all of them have stoves or generators.
I think the next most obvious option is warm clothing and blankets, which it seems like they have in good supply thankfully.
My org has done some building projects in the past and is considering doing more, possibly with a rocket stove type design, though I'm wondering if that's worth the effort, and our money/time would be better spent providing the residents with more food/clothing/fuel.
Check out this diy thermal battery. The guy shows a cooling setup first but there's also a heating setup. Pretty cheap to make lots of the material, not 100% sure what the best way of putting it into a blanket would be, but something to get you started.
I found a YouTube link in your comment. Here are links to the same video on alternative frontends that protect your privacy: