At the risk of sounding a right tight a**e. I take a plastic 6pt milk bottle, cut off the bottom, discard, cut up the side. (If you don't want huge pieces it's easier to just cut out the thinner sections). If they're not flat enough, sandwich between 2 baking sheets, put in oven with a weight on top & heat. Not too hot, around 100° if I remember correctly. When cool you have a marvellous material which can be cut (but not with dressmaking scissors). I've used this for bottoms of bags, circumference of cylinders, bags for children and the bottom of a doll's carrycot.