The counterweight on my reel crank has hemispherical shape. Sherline sells an accessory that turns spherical shapes, and it is just what you would need to make more than a hemisphere. But I have found that I can get by with general purpose accessories.
To make a hemispherical counterweight, I first made a brass mandrel that holds the work away from the chuck and encloses the screw that will secure the work. I make a blank for the counterweight that is cylindrical and has a tapped, blind hole.
I put the chuck, mandrel, and blank on the mill headstock, turned horizontal. I then fasten a lathe toolpost to the rotary table. The toolpost stays at constant radius, and I use X-axis feed between turning passes. Each cutting pass is made by turning the rotary axis through 90 degrees.