VeroWing
GreatGrady Captain
I've done several with marine plywood and in a few years they started showing water intrusion on bottoms. On my last one (22 Seafarer) I used this pfc board, and it was cheap, weatherproof, easy to work with, and lasted years until I sold boat, and I'm confident it still is. The cabin bulkhead is not a structural wall, just a divider between cabin and helm.

Veranda 1/2in. x 48 in. x 8 ft. White Reversible PVC Sheet H120AWS6 - The Home Depot
Protect your hard surface against dents and weather damage by choosing this Veranda HP 1/ White PVC Trim.
www.homedepot.com