Do you need a backrest support, because if not then just a luggage plate and your own bag. I needed to provide a back rest for my wife. I wanted something similar, waterproof, so I ended up with a Hepco Becker Gobi. Go to my images and you can see better pictures. This was not a budget solution. Just shy of the Ducati top case. With mount and box $530.00 USD.
I like it, it works for my wife, and when I use it alone its good for over night or a helmet box (some helmets will not fit in this box). Tough for just about anything, has stayed on the mount at speeds up to 115mph alone, with wife 111mph. Have not gone faster with the box on. Its rated at 80mph, Ducati top case is rated at 111mph.
It is surely waterproof, tested in torrential downpour 2 hours, bone dry.