We love our rice bags! Aches and pains, cramps, stiff shoulder, cold feet, you name it and our bags get used.
I've made several shapes. Squares about 8" work well for knees, rectangles work well for the back or feet. For neck ones, I sew channels into a longer, more narrow tube so that the rice doesn't end up all in one place.
If you use field corn, make sure you get cleaned corn (this has had the loose chaff and small pieces of dried cob) to reduce possibility of fire.
Using muslin, I make a bag the size desired, then make a pretty outer case; this makes it easier to keep clean. Your fabric is very pretty!