First, pencil in the location of the hatch. Second, cover the area of the hatch with transparent monocote. Next, do a layup with multiple layers of glass cloth. Let cure. Use a Sharpie pen to trace the hatch outline onto the layup. Pop off the layup and cut the part to the size. Now you have a hatch that exactly matches the contours of the fuselage. You will need to devise your own hold-down method. I glue ply strips in each corner and then use 2-56 screws to hold the hatch in position. Regards.