Plaster sphere
A reasonably accurate sphere can be made easily by hand from wire mesh (chicken wire), scrim and plaster.
The wire is moulded roughly into a size slightly smaller than the intended circumference, scrim is tied around to hold the plaster which is then applied as it is starting to set, but still workable.
See object Once the size is achieved it can then be worked with simple abrasive tools and finished with waterproof abrasive paper.