An easy thing that comes to mind would be to simply epoxy some cross bracing on the underside of the hatch before cutting it out. You could do this prior to cutting the hatch by marking your hatch cutout. One way to do this is to do a partial cut out of the hatch so you know where to postion the braces. Small holes drilled at different spots along the cutout line would also accomplish the same thing. One caution: make sure that your braces are set back enough from the edges to clear the hatch lip. Installing the braces before cutting out should prevent any spring back