Thanks alll!
Diane, I used Soft and Stable in the lower part of the bag, and a stiff interfacing in the flap. Matelasse (not too stiff, but it is a double-layered fabric) for the outer bag and quilting cotton for the lining. It was such an easy project. I serged across one edge, folded it up, and then serged around the "U" of the whole bag, using decorative thread in the loopers. (I did attach a sectioned pocket to the inside and installed a magnetic snap before sewing it up.)
BethAnn, you are too much! Marcy is such an inspiration.