Went and visited a shop run by a guy I occasionally surf with, he showed me a new model he's started to make with a quad fin setup , he reckons another mate of ours has one in a diamond tail that he uses as a quad most of the time, he says it's incredibly loose.

This on was a 9'4" x 23 x 3.

Looked pretty standard apart form a noticeable double conc through the tail.

Not really my cup of tea, but at least it has the option of going to a single.