I just replaced one of those JC Whitney (genuine Chinese) top boxes with a Givi 55 for the extra room, and so that the JC W. type didn't break the striker off and commit suicide on the highway.
There are a couple of ways to mount one of these to a C-14 but there is also some question about the strength of the mounts. The Givi also is about 2 inches more forward than the JC W. type was given the same mounting bracket. So I decided to make a stronger plate that would also mount the Givi further toward the rear to give my wife more room. The plate is 3/8" 6061-T6 aluminum, and has a S.W. Motech top box mounting kit adapted to it. The Motech kit provides the forward pins to mount the Givi, as well as the rear mounting tongue and four rubber bumpers to provide a place for the box to rest on the plate as well as stopping the box from rattling. My design includes an additional pair of bolts forward of the main luggage rack mounting positions and they make the entire mount considerably stronger without having to use external supports like the Givi 55 box mount itself does. Of course the plate had to be modified to fit everything as most prototypes do. But the plate is quite strong and fairly light (4 lbs.) and I am pretty confident it will carry anything a rational person might want to put into or mount onto that Givi top box. I also have plans for things to mount below the new plate but that will happen later.
Kirby shown just for size and age reference: Kirby is small and very old, the rack plate is new and medium sized.
A quick stress analysis showed that the angled slots provide much better resistance to the rack wracking than straight slots did. They also provide more bending resistance at the mounting tongue, where the box will load the plate, than smaller, straight slots.
A coat of semi- flat paint will finish the plate, make it much less visible and protect it from corrosion.
Brian