Yet Another Transom Job in the Works - ?ions

[Fishjack]

It's been a while since I have posted - I have been tied up with all kind of hurricane Katrina work here in the middle of "ground zero" Gulfport, MS. Things are a little closer to normal and I want to start on my transom replacement project. I plan on totally closing in the transom and doing away with the in deck rear live well.

This is where I need my 1st piece of advice -
The only hardware/marine supply store that stocked marine plywood was located in Biloxi right on the bay and everything got washed away including their extensive stock of marine plywood.
Shipping on marine plywood is really expensive if I ordered it online or elsewhere and had it delivered.

There is one locally owned marine store that can order the "Penske Board" by the sheet. They don't stock marine plywood and cannot order just a couple of sheets of plywood.

Should I stick with my original plan to use marine plywood (2 pieces of 1/2 inch) and just plan on driving to Florida or Louisiana to get it or substitute the Penske board for marine plywood??
I can get the Penske board for around $100 per sheet of 1/2 inch. I have done my research here on the site and have seen the Penske board mentioned a couple of times.

I am going to do the job from the back of the transom. Any advise would be greatly appreciated. And I'm sure I will have more questions as the project starts.
Thanks in advance.
Jack