These boxes aren't cheap----side opening if used as saddle bags, in my world gravity puts you stuff on the ground each time you open.
#2 good solution: Twisted Throttle, I think Fehling racks are available, that take Givi boxes----E-21 top opening, cheap, light, GIVI quality
#3 Top Box----none beats GIVI, MY E-46 has now well over 500,000 miles, including the shaking of Harley Sportsters. Can be mounted on Burgman with no GIVI rack, just bolted on. Big fender washer spread the load around bolt holes in box
#4 Real cheap and satisfactory solution----yellow Stanley tool box, 24" wide @about $15 bolted directly to rear wing/spoiler---great to be seen. I have 2----big one and small one.
One thing to consider-----any box mounted behind rear axle center=not a plus for handling----stuff inside really jumps around, possible beat itself to death.