I really like the idea of a three-seater version too. I've been thinking it would probably be better to go with a totally new single-seatar chair rather than try to modify the current bench. Upside is that you can put in a higher-quality chair, but the downside is that the standard set of front and rear seats can be laid-back flat to make the entire length of the car a bed - with a single third chair, particularly an aftermarket one, it is unlikely you could the same level of comfort.