There are several threads on this forum with photos and such.
Givi and Hepco & Becker make specific mounting kits to fit the R. Not cheap. And not exactly pretty, especially when the box is removed.
Several people have adapted universal mounting plates to the clamp on to the factory "luggage" rack.
Ventura Aeropak makes a kit to mount their soft luggage, which I have used in the past and love. Stores tons of stuff and makes a great backrest (even though their disclaimer says not to use it as such). But their mounting brackets for the R1150R/Rockster won't fit if you have the System Case brackets.
I've been tempted to buy a set anyway and fab my own mounting brackets. Seems like it would be easy to adapt something to the fit where the factory "luggage" rack mounted.