Dave,
I'm building a FS but I have built a few before, a few with fixed flaps and a few with movable flaps.
Simply draw a center line through the airfoil opening on each side of the fuselage. Carry the line far enough to go beyond the width of the flap.
I'd put the wing in place permanently then the flaps. You can tape them in place, then hit them with CA.
Do a side at a time.
Unless something better comes along, this is what I'll do.
Was your LE or TE warped? Old kit or a repro?
I have an OS 20 for mine, what will you use in yours?
CB