I learned about this method when I was pregnant and had the flu. I read that heat on the abdomen would help reduce stomach spasms and, in turn, reduce vomiting. I tried it. It worked!
Take a small pot and pour rice in it. (I don't use my organic brown basmati rice, though. I grab an old bag of white sushi rice that I hardly ever use.) Turn the burner to medium and shake the pan several times. When the rice is hot, pour it into an old pillow case, fold it over a few times and press it to the area needing heat. As the heat begins to dissipate, unfold the pillow case so there are less layers between it and you.
Surprisingly, the heat lasts for quite awhile and is super easy to reheat, by simply dumping the rice back into the pot and reheating. There is now always a pillowcase filled with rice in my medicine cabinet and I'm grateful for the cheap and easy heating pad I always have on hand.
The rice is dry and so is the pot - we're not cooking rice here, just heating the dry grain.
Electric heating pads emit EMF's and may not be a great choice.
Microwavable heating pads use a microwave, which emits electromagnetic radiation. It wouldn't be my first choice, even if I had one.
Rice + pot + pillowcase = quick, non-toxic, and practically free!