We have dry fitted the chainplate backing plate for the starboard forward mizzen stay. We measured the deck thickness at the old diesel fuel filler at 20mm. With this backing plate of 9mm immensely dense, stiff and strong FR4 board it makes me feel that having an additional 2 layers of 9mm plywood in the aft cabin was very much overkill!!
Next visit we will repeat for the port side and then make permanent with thickened epoxy (so that it is bedded to the underside of the deck/hull joint with no gaps).