You can make a pattern by first making a compass (yard stick with pencil taped crosswise solidly) then following the curve of the hull onto a piece of cardboard (with the compass) from OUTSIDE the hull.
For the stip to mount this to, notch the wood so that it'll bend to the shape of the hull, glue it and brace it until the glue dries or kicks.
It would be easier to shape the shelf and then tab it into place with epoxy ($$) or polyester (less $$) and strips of fiberglass matt, heavy cloth or light roving.
I don't know what a ledger board is.