After straightening the form the transom sometimes STILL
have a bit of a twist to it. Run a wire from the top of
the stem to the far corner of the transom. Twist the wire
until top edge of the transom sights straight and true
with the top edges of the temporary ribs.
Leave this wire in place while installing the bottom panel.
Later, after flipping the boat over, leave that wire still
in place until a rear seat, rear deck or rear dry storage
locker is installed. That will hold it square thereafter.