G.B.
You can't wash rice. You do need to google weighted vests and other items like this. You can buy weights right on most of those websites or get an idea of the materials that would work for what you need.
One thing "I" did when I made a weighted vest was put a bunch of pockets in it. Then I could take out what ever weight I used and I could wash and dry the vest in the regular washer and dryer.
If you use a light weight fabric like plain cotton to make the blanket it will be cooler, man made materials reflect the heat back on you. I will say though, wash the fabric in the warmest water you will EVER wash it on. Then dry it on high. This way it won't shrink at all once you have cut it out and made your blanket.
So many seamstresses skip washing and drying the fabric and then are disappointed when they use their item and wash and dry it. Then it no longer fits or is the right shape.
You could even make bean bag pouches that fit in the pockets and use them for games too....lol!!!!