I wish I could find that stuff. I used paper based corrugated cardboard from pizza boxes. Works great but I dread the first time I use that with an all day rain. I did have a couple of drizzly days on my last tour, but not enough water got thru the fabric to weaken the cardboard. The sides of my Nelson Long Flap sit on my panniers nicely, but I need the stiffener to support the middle of the bag where there is no underlying support.

Note to Burtsbees, you might try using what I used (paper based cardboard) to see how it works and to get your size nailed down. Then once you are happy with size and shape, use the good stuff cited by adventurepdx. I used a pizza cutter wheel to put creases in the cardboard where I wanted it to fold.