The best material to make a rice bag out of is 100% cotton fabric. Cotton is natural, durable, breathable, and heat-resistant which makes it ideal for creating rice bags that will be heated in the microwave without any safety concerns. It allows heat to penetrate through while preventing burns on your skin or damage to the bag itself during heating periods. Additionally, cotton fabrics are available in various colors and patterns which allows you to personalize your rice bag's appearance.