These cups have been made with a 60 degree taper in side to fit live and dead centres. The other end is a half round rebate to allow the sphere to sit flush on the edge of the opening.
The product is from high quality HDPE and does not mark the project although we have used soft tissue to help when polishing to ensure no marks are transferred.
With the product being HDPE it allows for turning of a tenon to fit in to chuck for the headstock side. We purposefully didnt make a tenon on the parts due to large variety in chucks and jaws available.