So I'm going to show you how to make your very own microwaveable heat-up bad in under 10 minutes if you've got access to a sewing machine.
First of all take a cheap length of some cotton fabric and cut a rectangle out of it. Size doesn't matter, you can chose to make a big bag or small bag.
Step 1: Sewing it up
Fold your rectangle in half and sew two of the three open sides shut. This leaves you with a handy dandy sack for your rice. But that's not all there is to it, first reinforce the sewn sides with a zigzag as you don't want rice escaping. Then turn the bag inside out.