Having a steel coffee cup that stays warm without a battery powered heat source indicated to me that using hand warmers placed into an appropriate space in the bottom of the cup might work. Someone would need to manufacture cups designed to accept a hand warm to keep a coffee mug warm for 18 hours, or possibly design their own specialty shape of iron filing based exothermic heating inserts to market.
That led me to discover that it is feasible to make hand warms at home. https://learning-center.homesciencetools.com/article/how-to-make-homemade-hand-warmers-science-project/
