Last time around on a S&G I did something similar. I primed and glassed the underside of the whole 4 x 8, and immediately bowed it into a half-cylinder with duct tape run from long edge to long edge, approximating the camber of the deck but just a little more severe (to allow for relaxation of the bend when the tape was removed). After a week or so to allow for hard cure, I cut out and installed the deck, and glassed the topsides. After another week, I cut out the hatch. The prestressing, double glassing and hard cure made the hatch fit perfectly with no ribs.