I would definitely not use flannel because of the flammability. Better safe than sorry. Additionally, the bowls often times don't keep their stiff shape after being washed, even though they have the all-cotton batting in them, like Wrap & Zap. If you did it with just flannel and cotton, I don't think you would be very happy with the shape after washing. Additionally, the whole point of these is to keep the heat from burning you. Only using flannel and cotton would be not be sufficient to do the trick.