I don't know about what grabber claims, I do use them.
Without searching I feel there must be something that will work as well as the rice and possibly last longer. I'm guessing the dampness has been cooked out of the rice and there is nothing to heat anymore.
I have a hydroculator pack, like a hot water bottle only filled with something like sand,from the hospital that can be boiled or microwaved I think. If you can find out what is in one you might be all set for a long time. I bet google will tell you.
Edit: Here you go, different kinds than can be hot or cold.
http://www.hbwsupply.com/index.html?...d_Therapy.html