Vincent that is some mighty fine joinery and I agree about the sanding especially uniform ribs, without them you will never get a uniform skin on. I never use magazines or bags to hold skin down (I know many do) I do the same thing you do, use bars because there is nothing straighter.
I was surprised you didn't put the sheeting on as one piece by taping the joints, fold and gluing, how did you manage to keep that joint straight with some of it on the wing, "you are really good".
Leroy