I actually use a variety of items rescued from the trash, most of the time. There is 1 kind of cardboard box that has smaller 'corrugations' in it than the regular shipping boxes. I cut 2 pieces to the size I need and cover it with lining fabric. I've never had a purse bend and it seems to work fine. Also, I left a cutting mat in the car and it warped badly in our Texas heat, so I cut it into strips to use for purse bottoms. Sometimes I use that plastic canvas if there is a large enough piece left after the original use, and I've also used foam poster boards. All of them have worked for me.