: Here are some ideas you might try:

: Hardware stores sell things for taking a pattern from a molding,
: usually used for cutting floor tiles to shape I believe, it
: looks like a bunch of short wires in a frame.

: Another idea is to use a laser level, the type that projects a
: line; shine it at the side of the boat and then take a photo
: looking at the bow along the centerline. If you include a scale
: in the picture you should be able to print it full size.

: Cut a rough outline in ply and use Plaster of Paris or drywall
: compound to fill in the gap and "correct" the shape.

: Be aware that the 2 sides of the hull may not be exactly identical.

You can also take a piece of cardboard, cut it to the shape that fits where you want to install a bulkhead, and use it as a template for creating the plywood or other material used to make the bulkhead.