we started making rice bags about 10 years ago- i use long-grain white rice (i buy it in bulk- inexpensive) we tried a couple other fillers (dried corn/ beans) hated the smell when it was heated- went back to rice.
i always make them in muslin first then cover with what ever-
the ones we made the granddaughters i made removeable covers for - so they can take the muslin bag out and wash the flannel cover. they stay warm a long time- i also keep one in a ziplock in the door of the freezer for when a cold pack is needed- works equally great for that.
my son in law is in the Army- and the first time he was being deployed we took pictures of him holding/hugging each of the girls- printed the picture on fabric- for each girl- and made them rice bags-so when they are missing him they can warm their bag and have a warm-daddy hug to snuggle with-
he has one too that he takes with him that has all 3 of his girls on it - he gets to missing them too :)