I'm using the hatch cutout-section of the deck to make the hatch. This seems much easier than making a hatch from scratch.
First step is to glue some foam round the edges to get to the required size.
After that I made a mold to laminate the sides and aft side of the hatch. This leaves me some time to think of a way to mold&laminate the front end of the hatch.
Sides & aft-side laminated. Finally something other builders might consider: in the plans the deck stringer is positioned almost against the hatch coaming. That may be the strongest solution, but might be difficult to laminate & get fair. After seeing the picture on the site of Oliver Doms of the coaming-stringer edge I decided to make the hatch cutout 1 cm less deep to get a bit of working space between the coaming and the stringer. Guess this won't hurt the 'integrity' of my boat.