Thanks everybody for your comments re additional boxes. They are plastic and the frame is aluminium, so I had considered the weight, stability, leverage on the frame etc, however based on the comments I will abandon the idea because of the issue raised re insurance. It Sod's law that if I headed off on holidays to Europe and something happened, I might not be insured, as I had modified the CT. So it's the back pack for pillion passenger and bunji cords to strap on any additional gear ,if I don't get a better, larger Topbox than the Honda one that I have. Ta O.