It seems the lower fin channel rib tends to get distorted when the fabric is heat shrunk. I placed an unused, (read miss cut) channel rib upside-down inside the fin rib. This formed a type of box beam for reinforcement. The new piece doesn't need to be the full length of the fin rib, 60-75% should be fine. pop in a couple of rivets on each side and it's done. This lightweight addition remained straight when I cranked up the heat to shrink the Dacron. Sorry, it's not the best picture.
