Don't have a good size reference here, but these electric hand warmers are smaller than I expected. Have to charge them up and see how they work.
hmmm, a bit larger one, or maybe two of those, would work like those Japanese devices they used to use to keep warm.
metal boxes filled with hot coals and wrapped over the solar plexus with a wide belt.
similar to the foot warmers used in beds back when.
@poemblaze
right, that too. heated rocks for sauna and sweat lodge too.