I just ordered starboard from this guy. Decent price and got it quick.
King Starboard Products And Prices
And I was able to get it in a color which closely matched my interior gelcoat. But this would not make a great seacock support plate.
As for the plate. I made mine to match what Pacific Seacraft does by laminating seven layers of cloth in a waxed plastic bowl mold. Take a little more work but structurally sound, no chance of deterioration and looks good.