I know nothing about your Dahon case, but I can say that I have carried my S&S coupled bike in the S&S Backpack Case on trips before. And one trip, I carried my folding bike in the S&S case.
Backpack bike case for a folding travel bicycle
The shoulder straps are in a side pocket for storage. At the amazon listing, there are two photos. One with straps out, one with straps stowed in a pocket.
https://www.amazon.com/Backpack-Trav.../dp/B001PTDQPQ
The case has four side pieces that are held to each other by velcro straps in the corners. When you remove those corner straps, the four side pieces can be stacked together to collapse the case. I put a piece of Coroplast in the top and bottom of the case to add more structure to it, white in the photos.
That said, my case would be a bit large for me to want to wear it or carry it on a bike to the airport, even when folded up. If I did not include the Corroplast sheets, the four side pieces are still about 25cm X ~70cm and are stiff so they can not be folded. The rest of the case is fabric and can be folded if it is empty.
The first layer in the case:
The rest of the bike in the case:
Backpack straps stowed in a pocket to check as luggage.
The farthest that I have carried this case with a bike in it was about a half km, it was better than having to carry it by hand but it is not the most comfortable backpack to carry.
Good luck with your project.