http://s134.photobucket.com/albums/q...t=IMG_0913.jpg
I have a step-by-step instructional series of photographs. I use reusable grocery bags, and they fit
inside the Coroplast
tm Panniers. these Panniers are 12 inches deep, so the center of gravity is low.
http://s134.photobucket.com/albums/q...t=IMG_0913.jpg
Click on the link, and then click "next" to view the photostream in reverse order.
This is not exactly what you asked for, but you can put your bag inside one of these hard panniers, or use grocery bags.