That cardboard pattern is for the stern/rear "in-fill" bag which fits between the sponson tails.
The picture taken of the inside of the stern bag is prior to rivet installation.
I'm still developing this skirt, the stern bag's relationship to the side sponson bag skirt needs work. It tends to detach or open up on turns, or as shown in the video when I touch the curb. This allows vast amounts of air to suddenly escape - not a good thing. I did not want to seal this connection because allowing the tail to wag is part of my low wear concept. It will wear less if is allowed to move more.