Maybe a couple of good shots of the area that you want to apply this to would help.
If I understand correctly what you want is a shelf to fit the hull shape above the V berths for extra storage of personal items.
Was there nothing present when you bought the boat? Copy it even if in bad shape so you can get the shape?
I strongly suspect you will have to either come up with your own or hire a marine carpenter to make one.
The curve could be a problem but that can be dealt with the use of tick sticks and carefull work. THe curve will not have to be exact but it can be close.