I have a genuine yamaha luggage carrier for sale. Yamaha Part Number 5vx-w0736-00-00
I haven't used it but the previous owner of my 2008 FZ6 S2 was very proud of it! It is a yamaha genuine part that fits directly to the pillion handles. It is fitted with a monolock case attachment - but I have never tried fitting a top box to it. If you see the pics and instructions on the attachments then hopefully it will be clear.
Offers please, but bear in mind if it's a silly low offer then I won't bother to walk to the post office to post it to you! It will cost about £6 to post. Clear funds before it goes in the post.
What I don't have is the protective strips that stick to the handles to protect them. You can use tape or thin rubber.
If you open the PDF attachment - it shows the mounting instructions.