Welcome to the Forum from Sunny South Florida. With a collection of bikes as you've described, it will be absolutely imperative for you to post pictures of them as soon as possible - we're a very visual group!
Funny you should ask that because I am preparing for a long ride in August and I was just about to mount that little set up. It is a fixture I made that fits into the backrest mount. It locates the Givi top box over the passenger seat giving the bike a more compact feel. I tried to send a picture of it off the bike.