It's interesting, on your set-up the opening in the rear latch faces forward, same as Givi monokey, however for it to work with the BESTEM it needed to face to the rear? I don't know if it's different for all Bestem trunks. Thanks for the photos.
The rear latch opening is facing rearward. The last photo is taken from the back of the bike, the lighting is pretty bad. It looks a little strange because you need that extra strip of material on the front side which keeps the whole box from sliding rearward and unlatching (probably why the Givi faces forward). I could see if I can get the CAD drawings if that would help, I'm not sure if my friend kept them or not.