We extend to you a cordial "WELCOME ABOARD !" Come on in, make yourself at home, we are a friendly group of enthusiasts, and we also appreciate the classic Chris Craft Roamer, Corsair, and Lancer boats too , as they are all on the same family tree and share much in common !
Check out the above site under plywood, look under "Cabinet Grade Plywood". We used the 6mm thick Philippine Mahogany Plywood, The standard sheet size is 4X8, but you can get 4X10 sheets for $59.00 each. The 1/4" 4X8 is $21. a sheet, so just get the big sheets for where you need the width! Marine ply isn't that much more expensive at $110. per sheet for the 4X8 6mm, and $120. for the 3/8" or 9mm but I don't know about bigger sheet sizes.
The folks at Westwind are really helpful, I'm sure they would reveal their source for the 4X10 sheets so you can track them down locally. They think this is suitable for headliner usage, they would not recommend it for decking etc.
The local lumber yard charges me $20 to deliver 3 blocks, Westwind sends it from Vancouver Island (20 miles and a ferry ride) and charges me $20. very reasonable company!
Check the fit of your panels before you cover them. The foam and fabric squish down pretty tight, so don't allow for a lot of extra space between the panels.
Good luck finding the bigger sheet size! They are made!