I like Pilou's method, but I would make the blend distance much longer.

Make one "C" curve along the sewn seam, with the open end facing downward.
Place the "random" wave along the C curve. Offset the C curve upwards and wider.
Flow the wave to the upper, bigger C curve. Sweep a line along the two waves. The bottom edge of the wave surface will be used for a blend, downward, to a 3rd C arch surface without any wave. It's top edge is the edge to blend to.

Well I still have to try this:-)

Brian