get a piece of hardboard about 3/4" x 6" x 18" and spray glue sandpaper on both sides of it - fine on one side and coarse on the other. Clamp it to your workbench and use it to flatten things like this.

Personally, I would sand as much of the wing as I could to get it flat. If I couldn't get it completely flat without losing too much material, then yes, I would laminate something to it that would allow me to sand it completely flat. I wouldn't use balsa though. Maybe some lite-ply.