I like the Givi/Kappa monokey brand.
Current favourite combo is a largeish topbox, an ancient E45 box with diddy E21 side cases. E21 are top loaders and great for clothing. E45 is big enough to stash a helmet if you are on foot for a while. This set-up provides more than enough space for a solo trip for 10 days.
I'm going to have this on my new R1200RS. If Givi don't make a fitting kit, have a look at SW Motech. They have rails that you can fit an adapter kit to suit most makes of boxes.
Kappa K40 side cases are okay and were very cheap on Ebay from Italy but the exchange rate may have altered that. They are clamshell boxes and capacious. Quite wide though.