I decided to 'wing it'. I knew that to make a ball I would need some tapered rectangles.
Six panels seemed like a good number, so I cut them out.
I then basted them together to see what happened. Not so round! It looked like if I added a couple more wedges that it would work, so I cut two more and basted them on. Now it looked reasonably round. I raided my trimmings box for bits to make tabs and handles and bumpy edges for baby to grab. Then I set about sewing pairs together with tabs and handles in the seams. Once I had the four pairs together, I sewed them to each other with piping and rick-rack in between. The resulting hole at the top and bottom was big enough for me to turn it right side out.
I packed it full of fiberfill and threw some bells in for good measure. (all things I had lying around, you know) Using some blue jersey circles for end-caps, I hand-sewed them over the holes on each end.