Another top box mounting option
I've used it this way, but there's still some cosmetic detail to finish. If there's interest I can post more detailed pics after it's complete.
I've had a top case mounted to a rear rack before. Carries the weight higher and further back and makes the bike behave worse in crosswinds. The only thing I'd used the pillion seat of my Versys for was soft luggage. So why not put hard luggage there too? I should be able to get a rider backrest from this as well.
If you like top box plus crap strapped to the seat, this way is limiting. I've never needed both--though I can see the point. For example climbing rope might be good to put on the seat.