I have not done this particular repair, but a few ideas come to mind.
First off, obviously the underside needs to be totally clean - dewaxed (acetone), sanded to the bare glass.
With the surface prepped you could:
1) epoxy in a piece of 1/2", or even 3/4", thick marine ply slightly smaller than the hatch underside (to make space for the cloth you'll put on over the top of it). Use thickened epoxy to adhere to the plywood to the underside of the hatch, which will ensure you have no voids. When it has dried, paint the other side of the wood with epoxy, and lay one or two sheets of fiberglass cloth to seal it over the wood and onto the hatch - this will ensure solid connection. You end up with a similar "sandwich" to what is used in the decks.
2) An alternative would be to put in stringers. You can use anything to form them - in a similar situation I used 2" foam pipe insulation, cut in half. Place the form where the strength is needed, and then lay up at least 3-4 layers of cloth, each slightly larger than the other, to secure the stringer to the hatch. As this sets it effectively produces a rigid fiberglass stringer. Biaxial cloth makes this easier.
Hope this helps - I'll be interested in other opinions. The plywood approach may sound a bit intimidating, but is actually fun to do and really solid when it is done....