It ain't curved.
It's actually flat, just screwed down to a curved surface, thus making it curve. Any decent glass place should be able to order tinted plexiglass for you. I don't know about the left coast, but I paid $70 for two 1/8" pieces about 12" x 24" over here on the right coast. You need at least 1/4", thicker if you can get it. For some reason, tinted plexi is much more expensive than clear.To attach it, it's easier to remove the hatch frame from the boat. I'd have the plexi cut about 1/2" longer and wider than you need. Position the plexi over the frame, line up and drill for, then install the two center screws fore and aft. Then using wood clamps, or even c-clamps, clamp the four corners down, thus bending the plexi to the frame. Starting from the center screws and working out drill for and install the remaining screws (try to use the old holes in the wood, you might want a slightly larger screw however.Once the frame's screwed down, I'd use a router with a straight bit with guide bearing on the end to cut the plexi down to size. The sides will be angled out (due to the curve) but can be sanded or filed flat.You might want to consider a thin bead of silicone or other sealant between the wood and plexi, but I'm not sure that it's necessary.Hope this helps.