I originally started out with the idea of screwing a 3/4" piece of plywood directly to this small work table in order to ensure a flat surface for the wing rib jig. 

That didn't work.  The plans suggest framing the jig with 2X4's.  Obviously written in a different era.  Even here in the Pacific Northwest, where the lion's share of building materials come from, finding straight dimensional lumber can be done, but those selling it are mighty proud of it.

So I ripped some 3/4" plywood into 4" wide lumber and used that to frame the jig.  It is good and flat, checked with my 5' level.