Don't think I've ever really seen a retro top box like the one you're looking for but you can try cover a normal one with leather.
You are right, a top box is not for this bike. I'm actually looking for side bag matching bike style, that can be removed quickly. In which I can leave this http://www.espacsafe.es/LidSafe , but can't find stock of the helmet bag anywhere...http://www.ebay.com.au/itm/like/121881920664?lpid=107&chn=ps
Don't ruin your bike with a big silly box on the back!