Tigerdude,
Ok, here is how I would cover the wingtips on this plane. Since the wingtip is flat it's actually pretty easy to do. First cover your wing panels. When you have the covering completely stretched and ironed down on the wing panel then cut off the covering so that you have a "flap" of about 1/4" or so. Fold this over the end of the wing and iron it down to the outside edge of the wingtip. Do the same with the other side of the wing panel. Now use a single piece of covering to cover the actual wingtip, covering over the "flaps" you iron down from the wing panels. Do it this and you'll have a seamless covering piece and the covering will be anchored tightly for the wing panels.

Ken