The form boards were just some old pine boards I had laying around. They're half-rotted already, but I think they'll be up to the task. I cut them to the inside size of the hatch so I can attach the sides directly to them. The curve was cut with a jigsaw and then cleaned up with a belt sander until it eyeballed fair. To obtain the curve, I used a "spline" to connect the two end-points and centre of the arc. The spline was just a thin strip of plywood that had a consistent grain, but a nylon tent-pole or any other consistently bendy item would have worked better.

The plywood I'm using here is meranti that I got from the local home centre. I'm using this stuff for a couple reasons: it's inexpensive, it is very smooth with no knots or checks, and it's easy to work. I've previously given this stuff a boil-dry-boil test and there was no sign of the glue weakening. When I got to the home centre, I found a piece where they had accidentally gotten a piece of teak in the plywood factory as you can see by the underside of the skin in the first picture. I think I'll leave this exposed on the inside.

As you can see, I've nailed the forms square to the deck boards.