Bowing the wing in the opposite direction while shrinking should do the trick. Even after the covering is shrunk, you can bend the wing, alowing some wrinkles to form, and draw them out with a heat gun. If it's not to much, say 1/8"-1/4" you should be fine. In my opinion, a LT-40 is one of the best flying trainers on the market, and a slightly bowed wing aint gona amout to a hill of beans! Finish the plane and have fun with it!