Now here is a Idea, I can cut ome play templates at a few stages along the wing (4 should be planty) glue those to  a base 3/16 inch balsa X 30 inches or so. Fill in the spaces with rectabgular foam pieces then sand to shape with a block just longer than the span between the plywood templates. Of course the challange is to get foam pieces the proper size with 3 sides flat and square. Never mind finding it in small quantities. I have to look and see how small a piece I can find as I only need around 60 inches of 2" X 2" foam.

hence why you can use expanding foam filler from spray can - just squirt it in between the formers and let it expand - then shape........its EASY......and you dont have to cut foam, find foam etc and a can of expanding foam filler is about $8............