I just bought a Piaggio top case for my GTS on eBay...the mounting hardware was included but it's not really clear to me how it mounts. I guess I could try the trial and error method or drive over to the dealer and see how they're mounted but I'm into immediate gratification. If anyone has any pics of how it's done, or if it's been posted before please let me know.
Of course, it's also possible that the seller didn't include all of the mounting hardware which might be the cause of my confusion.