I spot hot glued each rib (one at a time) to the lower flap panel. I then used a strip of thin sheet aluminum with 50 grit sand paper spray glue to shape the rib. I placed the upper flap skin into position and pulled the sanding strip back and forth across the top of the rib until the top panel fit the lower panel properly. For the center rib I hot melt glued a long stick to the sanding strip so I could operate the sander from the edge of the flap panel. This worked well and took about an hour to fit all three