I've made many for sale at craft sales and use cotton, lined with warm and natural then fill half full of dry rice. Depending on how hot you want it, anywhere from 1 - 2 minutes heats it up perfectly. They also stay in the freezer forever without losing their flexibility since there is nothing wet to freeze. Perfect for an ice pack. The warm and natural adds to the insulation keeping the cold or hot longer and adds softness so the rice isn't lumpy.