I made a bag with a fusible product called Inn-Control Plus....it's a layer of foam rubber with fusible on one side. It definitely made a firm-sided bag and I used it for the bottom too. But...my sewing machine balked at more than two layers of it (with fabric) and I had to sew slowly on even two layers. But it made a very sturdy and professional-looking bag. I used a pattern from Keepsake Quilting and got the foam stuff from them also but I think it's available from the manufacturer's website.