I think making the board out of fiberglass would be a great idea, Nicholas. It would be a lot lighter, and stand up better to the elements, than those made from painted plywood. You could likely market it after working out a good mould for it. As I am not set up for the same processes you are, I wonder about using a board made out of thin (1/8 or 1/4") Baltic Birch ply, and laminating it with glass. It would be a bit more time consuming than simply molding one, but as a one off item, it would save lots of time on the mould making process.
Take lots of photos along the way, I am eager to see your process and how it works.
I used to play this game quite a bit over the years, along with horseshoes, but in the past 15 years or so, we have mostly played all-terrain bocce, or Snake Toss, as they break down better for transport while RVing. The lighter Corn Toss board might make that more appealing once again.
In my books, any game you can play one-handed, while holding your favourite beverage in your other had, is tops in my books.