Rise from your grave!
I finally got a case on my scooter, after looking around a lot for a pre-fab rack, with no luck.
So I just took one of our used Ikea cutting boards from the kitchen, and drilled a few holes into it to take the place of the useless "wing" thing.
Bolted in using M6 bolts (can't remember length though, but I think around 35mm) with some nylon spacers and washers. The Givi/Kappa plate is bolted in with 4 countersunk M6 bolts from the bottom, and nylon insert nuts from the top. It works well, but the cutting board flexes quite a bit, so if I feel spendy I can replace it with something more rigid. But this altogether only cost me a few dollars in pocket change.
I didn't bother cutting or shaping the board at all - I can grab the hole in the board for pushing the scooter, and one other detail I didn't mention is that I also put two additional long bolts that protrude out the bottom - these I can use as attachment points for bungees if I need.