You "could" fill a sock with rice and then tie the top, but what I did was sew a rectangle pouch about 5" X 18" (leave one short end open so it looks like a pillowcase) and then put in 1 cup of rice and then sew (carefully so you don't break the needle on a grain of rice) and then add one more cup of rice and then sew. I put in 3 cups of rice so the pouch is divided into thirds. Then I made a little pillowcase like cover for the whole thing so I could wash the cover. I think you can put this in the microwave for 2-3 minutes and it stays pretty hot. I used this all the time during my last weeks of pregnancy, mostly on my lower back! I never tried freezing it but I guess that would work too. Maybe someone else here has done that before.
HThelps!
Liane