Andy
I think that the rack that fitted on the back of my Harley Ultra may do the job? I'll let you know when I get the bike in March.
The only thing that sort of worries me that the box lid is double skin, the painted part is a clam shell and may not take kindly to drilling and bolting. It also will make it very tall at the back end cross winds may cause a bit of weaving so thats the fancy luggage system out of the window.
I'm looking at having a rack made to replace the box when required IE camping etc.
Let us know how you get on
Chaos