stack up sheet balsa the correct thickness of the ribs. trace the rib on the top piece, stick 2-3 pins down all the way thru all the parts. cut them all out at the same time on a band saw (up to 3 inches thick or so). then use a sanding disc or block to bring them all down to exact size. Also cut any spar notches on the bands saw when they are pinned together. When you take them apart you will swear they were from a laser cut kit