my mom and grandma used to use sand. it doesn't expand or mold if it gets wet. it can be reused many times and it conforms to whatever you put it on. go to a craft store and get some heavy material and sew it to make a pillowcase. fill the case 3/4 full of sand and sew the last edge. make sure the stitches are very small to keep the sand in. these will last for years.
Hmm, never thought about using rice before, might try it out though. I've always just grabbed black beans, black eyed peas & even navy beans. I'd stick them in a gal. sized freezer bag, then just stick the whole thing in a t-shirt, that way I could wrap my knee