I posted the same question about a year and two months ago, no luck. I searched online as well. I ended up not making one, which was just as well for me since I didn't have the VBAC attempt I wanted.
The easiest idea I came up with was to cut a long rectangle to fit around the ball and wide enough to go from pole to pole, plus seam allowances. Seam the circumference, then put in casings on each end and gather them up with elastic or cord.
You could do orange segments with round pieces at each end like a beach ball too, but that seemed like a lot of seams to try to avoid with sensitive parts.
Post back if you make one, let me know what you come up with, OK?