Keep a look out on e-vilbay for a 2-4 seat, as they do crop up now and then......and it is definitely the closest match of those offered up. I know the picture of my 750 only shows it from the rear, but it is identical. Back in the 70s, 2-4 seats were very common on Honda 4's.
If you go to the "media" page of our site, and click "flickr" gallery, On the first page of pictures you will see a Gt500 with a 2-4 seat from a side view, and also a picture of my pal Tim's CB750, which has a 2-4 fitted.
www.classic2strokesuzuki.com