now hold one end of the melon between your index and middle finger, and the midpoint between your thumb and pinkie. very gently let the melon and arc stretch "into" each other. put pins in between.
if you pressed open your arc seams, it will go together with only a little fuss and stress.
do the same at the other end, except the midpoint is between index and second finger; end is between thumb and pinkie.
please note, the photo shows the pinned pieces from the back.