Outside ArmTo make a clean and straight edge on the top of the outside arm, we use cardboard tackstrip. See the result in the next image.
Front arm panelI decided to hand sew the front panels rather than to apply with other methods. A little more time consuming but hand stitching is nice slower pace during construction. You can see the crisp line on the outside arm from using cardboard tackstrip at the seam.
Updated ArmHere's a side-by-side of the previous front arm panel and the newly styled simple version.