Jim, congratulations on some fine thinking. Moving that bulkhead aft to where you need it, as opposed to the generic location given by the designer, will yield for you less flooded volume in the cockpit and increased space in the forward compartment. It is precisely what I did on my first-ever boat building project, and it is one of the proudest moments of my life (I don't get out much).

Any old plywood will do for those bulkheads. As long as it is epoxy sealed against moisture incursion, it's sure to be structurally sound. I'd use 1/4" El Cheapo from Home Despot or whatever. I found it a simple matter to plot the shape of my five-panel hull directly onto the plywood and let rip with the new part. A cardboard pattern ought to be easy enough to produce should you feel a bit timid.

Below is a photo of my hull with both bulkheads in place: the original, and the new one located 9" aft. You can plainly see what a wise move it was (okay, I confess it was Leslie's idea, dammit).

Cheers,
Kurt