Felt is polyester, which is plastic, so it will melt. If the floss is 100% cotton, it won't melt. When I make rice bags, I make the bag I put the rice in out of muslin, but then I make a removable cover out of flannel so it can be washed. Can you make a separate muslin or other cotton fabric inner bag for the rice and then just make the decorative part out of whatever you want? Be sure to include specific instructions with the gift. Also, a special note for rice bags, I include a small bottle of eucalyptus oil with rice bag gifts. You can put few drops onto the rice bag for extra sinus clearing power.