I have been making the bags, all shapes and sizes, for years, and I use rice with some lavender buds tossed in. I noticed that someone said they used bird seed and misted it. Wetting the rice is a good way to get a serious burn. I know someone who tried it.
I like the rice because it makes for a moist heat each time you use it. The best way to heat it is for a minute, roll the bag in your hand to redistribute the rice, then heat it more until it is a temp you want. The biggest bag I have takes about 90 seconds to heat completely.
I have made a couple with ties on them to wrap around my shoulders then tie to hold in place. They make nice gifts, and if you make them with removable covers, last forever.