I know it is a little late, but I have a seat off a 2010 H.D. Nightster that has a shape you may be interested in. Not being a salesman here as I'd like to keep it, just saying I could measure it up any dimension you need so you could probably find a take-off dirt cheap. Most Harley guys ditch the stock seat and exhaust soon after buying, so the chances of finding one next to nothing are good.
Mine looks just like this...
81 Honda CB750C - Current Project
67 BSA Spitfire MkIII - Next Up (Full Resto)
81 Honda GL1100 - Bob / CafeĀ“
80 Suzuki GS750L - Bratstyle
72 Honda CB450K5 - Basket Case
73 Honda CB350F Cafe' (Gone but not forgotten)
Don't wait for opportunity to knock... kick the door down and drag the old harlot in!