Rick,
I had the same concern about the hatch caving in under the weight of the sand.I used a couple of strips of wood bridged across the hatch and deck as reinforcement(the outside,upside down that is).I then taped the seam with good quality duct tape and/or clear packing tape. By the way,I have found that the cheaper clear tape or duct tape just doesn't have good enough adhesive to be of use in this application. I found the clear tape that U.P.S. sells is of awesome quality. As John mentioned you then need to use a fillet of thickened epoxy along both edges of the sacrificial weather strip in order for the cloth to lay over it nicely.With out the fillet the cloth cannot conform to the 90 degree corners when you pile on the sand. I used a plastic trash bag to cover the fresh epoxy and glass and it worked well. Saran wrap maybe,if it were strong enough. I did use a lot of wet sand to weight it down so reinforce and tape the heck out of the hatch before you begin. I used 6 layers of 6oz. e-glass and the lips are very stiff and strong. Hope this helps.
Kurt Hoffman