An ex-member on this forum, Rob Valdez got an OEM solo seat from an eBay seller SMeier ( Meyer ), or some spelling like that, from what I can remember it was for first gen bikes and the seat was rigidly attached, no hinged feature .
He also added a cargo rack behind the solo seat, made the bike more practical as an everyday use bike, without the usual limitations on what you could carry on it .
Rob left here about 3 years ago, at the time he was active on Boxerworks, don't know if he used the same name there or not .
He may be able to answer your questions, if he's still there .
Edit : I think he used Tom Farr as a username .