Windsor Plywood ( http://www.windsorplywood.com/ ) lists baltic birch ply in 3 through 24mm in 60"x60" sheets. You could laminate the 3mm stuff to get whatever thickness ribs you need, and you could get a lot of ribs outta 3mmx60"x60" !! For seventeen bucks ? That's a steal, down heah.

Now, if the chord exceeds 60" . . .
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