Contrary to the bottom rib flanges which are made of poplar wood, the other rib flanges are made of laminated spruce.  It requires a jig into which the spruce lamellas are glued. One for each rib as they are differently sized.

My woodworking skills are picking upo rapidly. Too little glue does not stick, too much glue ends up with a mess. The end result however fits nicely to the blueprint.