ORIGINAL: kdc

The easiest and most economical way of cutting paralell chord ribs is to cut a slightly oversize ply template without the slots and complex bits and use this to produce all the blanks. They could be about 1/16 oversize and cut roughly with a knife. They can be interleaved to get most out of a sheet and make a couple of spares too. Each should have two 1/8th inch holes to allow them to be bolted together as a pack for final sanding. Two pieces of threaded rod and nuts are used to bolt them together between 2 accurate templates with spar slots marked. Then place the pack can be gripped in a vice just below the threaded rods so no need to recess the nuts and sanded to shape with a sandpaper block, razor plane or preferably a Permagrit tool. Cut all the slot sides with a razor saw whilst still held in the pack and file to exact depth. The file will snap out the waste at the bottom of the razor saw cuts.
The photo shows what I mean....