Similar situation here, although I did not need to raise my Givi mount but put it back several inches since it was too close to the seat and driving two-up was impossible. What I did was simply to take a steel frame, mounted that one to my original mount (eight screws alltogether, including the two pins which required me to tap the original mount in two places) and mounted the mountpoints (two pins and the one for the lock) to said steel frame - voilà. Not only did it put my topcase back far enough for my girlfriend to be able to enjoy the ride but due to the construction it also raised it ~2cm.
I'll take a pic if so requested, it's easier to see for yourself than to have it described
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"