Cardboard is a pattern maker's best friend

Making a bulkhead

Measure back from the transom the same amount on either side to make two matching marks.
Stretch a board from from side to side, from mark to mark.
Clamp it down.

Make the boat level, so a 4' level reads level when placed over the middle of the bottom.



Then use a 4' foot level to transfer the edge of the cross-gunwale board to the floor. Now you're marked up and ready to go building a bulkhead.