I have a Givi E33 top box which is sort of small. I try to keep it empty so I have a convenient place to lock up my helmet and gloves at stops. I also have Tourmaster Cortech Sport saddle and tail bags. On long trips such as this, I replace the tail bag with a medium sized wheeled duffle bag bungeed to the passenger seat (makes a nice backrest, too, if you pack it right).
A word of caution if you don't already know this. The rain covers that come with soft luggage will blow off at highway speeds and are best left at home. If you want your stuff to stay dry, put everything in small trash bags before packing into your luggage. Zip lock bags work well for small items such as your registration and insurance cards, gas receipts, etc.
