Have you seen
http://www.altechdesigns.co.uk/11-luggage-racks He has 3 sizes.
I've just made my own by hand to fit an old hepco+becker topboks with a univeral tube rack.
H+B makes a special rack for it (€140) but it would but the box so far back that i would not be able to open the sidecases.
Yes i need to take it apart again and paint it (and they last 4 bolts has been put in with self-locking nuts below. 5mm alu is not much thread)