Funny thing, I was working on a quilted bag today. and I was extremely pleased with the heavy (thick) interfacing I used. Fusible would have been even better. The bag is holding it's shape really well. Can't remember what it's called, but it's about as thick as two pieces of flannel and much stiffer. Especially after I free-motion quilted all the pieces. I did not put interfacing in the seams. I cut it the size of the pieces without seam allowances so those seams would sew together easily. I interfaced the lining, then quilted it, then interfaced the fashion fabric then quilted it, etc. After preparing all the pieces, I began assembling the bag. So, my advice is...don't skimp on the interfacing or batting.