Hot/Cold microwavable bag
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.
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.