This hot/cold bag is really easy to make. It needs to be microwavable so use cotton only. I don’t know if any other fiber works but I know this one does for sure.
With a G hook, chain 40.Join to first chain to form a ring. Careful not to twist.
Crochet in the round and single crochet until the tube is long enough to wrap around your neck and shoulders. Flatten tube and slip stitch to close one end of the tube. Cast off and weave end. Fill the tube one third of the way with long rice (not the instantaneous variety). Slip stitch to close the other end of the tube and weave in loose strands.To use, either put it in the freezer for a couple of hours for cold. To heat, microwave at 30 seconds interval until you’ve reached the desired temperature. Avoid direct exposure to skin if too hot as it will burn you.