Here is what I found out.
The Givi rack consists of two components: the top case arms (Givi 265fz or the identical Kappa kz265) with the mounting plate for a Monokey top case (m7 or m5), and the mounts for side cases (Givi PLX 204). The side case mounts go on the top case arms and are optional, but the top case arms are not (some people who only want side cases have cut the rear portion of the top case arms).
You can still buy the PLX 204 from Givi, but the top case arms are hard to get. I just discovered a single exemplar in a store in Boston. It also seems as if sportbikeshop.co.uk can still source the top case arms.
Mounting the top case arms requires taking off the rear fairings and drilling two small slots through the black plastic (no big deal, but still). The Givi arms also go over the back lights (the back lights are visible, but it's a compromise).
Slightly more elegant might be to get Honda's "Cruising Backrest Mount" (08F75MEH800A), if you can still find one anywhere. It screws below the pillion seat. Honda has made a small top case with a backrest for it (it's too small for a helmet though). You might want to get the Honda VFR or Honda CTX top box, drill holes and screw the respective mounting plate directly on the Cruising Backrest Mount. The VFR and CTX top boxes have a good shape and can sometimes be found on ebay, even in Candy Apple red. But I don't know how much weight the Cruising Backrest Mount can hold.
The final option is to weld your own carrier rack to screw below the pillion seat.
I have installed the Givi top case arms and am ok with them. I will not buy side cases, because they make the bike wide and apparently affect the airflow.