make the hollow portion V shaped so a sphere of any side would fit in it. The way I've always done it before was it was a cove shape so only the outer edge touched the wood.
I want the ball seat against the rim of the drive cup. This gives a better hold and drive than a cone shape.
the cone has negatives the ball will always be on a smaller diameter drive radius than you want, the ball would be more likely to slip when being cut, and anything below the rim of the drive cut cannot be turned,
one of Don Doyle’s (rubber chucky) prototypes was a cup with little rings on the inside so that you had different size rims to seat against. This took away too much working room on the smaller balls.
I turn out a bowl shape not quite as deep as a hemisphere. Don’t want the ball to tough anything but the rim.
Ideal diameter is about 1/3 of the. The most secure drive would be just under 1/2 the diameter but there is no working room on the ball. 1/3 is a good compromise between hold and working room.
I soften the inside rim to a gentle curve.
I also like to use soft maple, mahogany, Camphor for the cup. These are woods that are softer and less likely to mark any balls I turn. They might leave a ring mark on Camphor but it goes away after sanding with 220.
plastic may be too hard for a good drive cup.
You can always put foam on the rim,
I use PVC vacuum chucks on large balls. I use the foam with them both to protect the work and improve the drive. in working when Don to perfecting the hardness of the ball chucks he made a couple that too hard and a couple that were way too soft and would flex and then he made it just right.
it’s always fun to experiment with materials but it’s hard to beat wood. I have been using some of the same wooden cups on wood MTs for 20 years. I rarely have to true the rims but I do check them every time.
wood chucks used in chucks need to be trued a lot.
I have a bunch of rubber chucky cups but rarely use them for balls. I do use the a lot for holding the seed jars I have been turning the last few years.