i make mine in muslin (pouches) then place the muslin filled with rice inside a removable flannel cover. we have been making/using them for 5 or 6 years now. i keep one in a zip-lock in the freezer for an instant cold pack when ice is needed and one on top of the microwave for heating. my granddaughter's each have one with a picture of themselves each with their daddy...so when he is (deployed) and they are missing him they can warm up their snuggle-bag and have a nice warm daddy hug!
ALWAYS USE COTTON FABRIC...POLY WILL MELT/CATCH FIRE IF OVER HEATED! always include a tag saying to heat for 1 minute...don't want anyone getting burned.