I made the cockpit rim first, and traced a cutout. I had the idea of making a plywood insert, but it was too hard to bend & shape to the hole.
This is what I went for instead-a whole bunch of horizontal strips. It was fairly easy.
After all those little strips were in place, I sanded it all down, cut out a rough opening, and fiberglassed inside & out. It takes out the across-the-boat curve, but there is still a considerable fore-and-aft curve near the back of the cockpit.