To construct the hatches for B3 I epoxied the stiffeners that I had previously laminated out of 2 layers of 1/4" baltic birch plywood. First I had to clean up the glue squeeze out and round over the corners on the router table.
The 9 mm ply cutouts had a twist in them and would not lie flat, so I took this opportunity to try to correct that. I put spacers under the low corners and clamped down the high corners. This flexed the plywood in the opposite direction of the twist.
I then applied thickened epoxy to the stiffener, and clamped it down to cure. After removing the clamps I found that the hatch now had a slight twist in the other direction - my spacers were a touch too thick. But it was much closer to flat than previously and the hatch latches should be able to flatten the hatch for a good seal.