I decided to buy a Givi Tanklock bag for my Ducati Multistrada, and purchased an extra ring for Honda (Part Number BF-03) It is not listed to fit the Honda 919 however I am here to tell you it does in fact work.
That's cool. How does it attach and lock to the ring?
When you buy the bag you get a plastic locking ring, then you buy the corresponding metal interface ring to attach the locking ring to the tank. the metal ring for Honda is Givi Part Number BF-03... the 919 is not listed as a compatible model, but for $15.00 i took a chance, and it worked beautifully.
the bag literally sits on the lock ring and latches with nearly no force, it un mounts by rotating a red lever... works perfect, its stable, and comes in multiple sizes, and has a cell phone pocket so you can use your devices built in GPS and have the screen visible.