I started with a cardboard cutout that I traced from one of the forms closest to the bulkhead location.  I cut it slightly small, taped it in place in the hull, and then used a carpenter's pencil held against the inside of the hull to trace a line onto the cardboard.  Then I transferred the line to the flat panel using the same offset.  The idea was to trace an exact outline of the inside of the hull.   In actuality, it was off quite a bit and took a lot of shaping to fit properly.