This is a interchangeable thumb slug for bowling ball that is optimized for 3D-printing. It will fit on the Switchgrip-mechanism.
I have used PETG-material to make this and it works great. In case of using some other material, it may require some tweaking for the clearance at the bottom mechanism. There is no thumb hole included because it will be different from person to person. Also the height of the slug should be adjusted.
Print the thing from bottom to top. Then you will make the top surface to accommodate the ball's surface.