Make an inside muslin bag and make "pockets" for the rice if
you want it to stay evenly distributed. If you don't care, or want to have the rice form to a body part, don't make small pockets. The outside cover can be made with fleece or cotton, sewn shut on three sides and velcro'd on one for easy innerds removal. I have used both rice and buckwheat
for fillers. The fleece will give you more of a moist heat.