"The only thing I would say, to finish, is that IT IS PROBABLY UNWISE to cut out the hatch openings (especially if they are large ones) before the outside is glassed. Glassing them separately may cause them to distort to a different shape to the deck and ruin your seal. I can't be sure, because I have never tried it, but I would be willing to bet on it."

Oh yes, they do :-(

However, if you force them back to the correct shape (I screwed them down to the original forms) and then heat the outside to about 200 C, then when they cool down they only spring back a very little.

Justin