That is a fascinating quilt! I would get a piece of upholstery plastic and draw on it with erasable marker, to audition designs. Three rays that widen to the center and then narrow back down to the bottom point of each piece might look good. I would stay 1/4" out to avoid seams. Or you might come up with a simple motif for the center of each piece that would follow its shape. Browsing on pinterest might give you some ideas.