https://www.amazon.com/OCOOPA-Fast-Charging-Handwarmer-Rechargeable-Activities/dp/B08GQ25V7D/ref=sr_1_20?c=ts&keywords=Camping%2BHand%2BWarmers&qid=1645929712&s=outdoor-recreation&sr=1-20&ts_id=14329821&th=1
You could check this book out. It gives some good things to think about, not necessarily a "how to" for specific trades though.