Simple solutions to single-use hot/cold packs and hand warmers are available in most kitchens. These reusable options reduce trash and the need to continuously spend money on disposable, one-time-use products such as heat wraps and instant hand warmers. Additional benefits include mobility (no cords), repairability (no electronics), and enjoyablity (no condensation or leaks).
Choose a natural filler such as rice, dried beans, lentils, etc. (Optional: Adjust the aroma by adding dried herbs or experimenting with different fillers. I use dried lavender petals.)
Find pack materials (worn-out long sock or salvaged cloth & thread/string/ribbon).
Make pack, fill and close.
Microwave 30 seconds to 1 minute at a time or chill in the freezer. Use as you would with a heating pad, heat wrap, hot water bottle, ice pack or hand warmer.
If using a sock, fill. Tie closed using the long cuff.
If using cloth, fold a square shaped piece into quarters and cut into quarter-circle. Unfold, put filling in center. Tie closed using a string or ribbon.
If sewing, fold cloth in half with the inside facing out. Sew two sides, then turn right-side out. Fill with filler. Stitch the remaining side closed.