there are a number of ways to attach the melons. one way is to use the same method we did when attaching the arcs to the melons. that is to pin at the center and ends, then pin the heck out of it in between.
in the photos below, i started in the center, then pinned outward toward each end. it's only partically pinned in the photo but ... if you proceed carefully, you'll find that the two pieces go together more easily than you expect and things line up at the ends.