Well, the MDO showed up… I don’t think I got the Olympic Panel Products brand that I wanted, but I’m going to go with it for the frames. By the way, the Olympic Panel people are great! I fired off an email and promptly got an answer referring me to one of their staff that was knowledgeable about my questions. He promptly emailed also, and provided phone numbers and I had a good conversation with him.
I also want to compliment Sign Supply USA, Inc. for their good customer service and being generally nice to deal with. I have emailed them and called them, and now my wife has dealt with their delivery driver, and all the encounters have been pretty good. They have offices scattered all over, so you might check them out.
Anyway, I have 5 sheets of 4×8 plywood, pre-primed with a tough seeming light grey primer. Edge voids are a bit more than I wished for, but this is a good bit cheaper than the true "marine ply" and I think it will do well where I’m planning to use it.
After I cut this all up and have a bunch of frames and bulkheads, I’ll know more what I want to use for the hull panels themselves.