[scook]

No matter how big or small you cut the plywood, do what Smoke says - cut the ribs (little beams) to conform to the underside of the deck.

Doing this I'd called scribing. You either wedge the rib in place (just enough tension to hold it there) or have a helper hold it there. Since the deck humps up, the top ends of the ridge will be touching the underside of the deck and there will be a gap in the middle. Measure the gap, cut a small block of wood approx. 1" to 2" wide by the height of the gap and use it to space your pencil down from the underside of the deck. Holding the rib in place, mark the curve of the deck onto the rib, cut the curve in the rib and hold it back in place to test the fit - make adjustments if necessary and you have a rib that will make the plywood conform to the deck.

The rib has to be stiff/strong enough to hold a bend in the plywood without bending. Two things to consider: 1) plywood will only bend in one direction, not both directions at the same time (a compound curve). If this is a problem, you will need to cut the plywood - it probably isn't but you can tell easily. Drill,the holes in the plywood before you dry fit it to the deck - if you can't get it to go tight to the deck, that's probably the problem. 2) to make the plywood bend more easily, you can score the back (top surface, the side that contacts the deck) perpendicular to the direction you want to bend it. For 1/2" ply, cut a little less than 1/4" deep, 1/2" to 3/4" apart and feel how it bends. If you need a few more scores, you can add them. The resin,will fill the grooves and you'll still have a strong deck.