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There is really no need for drilling. You can make any kind of hole already to the slug. Leave it unsanded and you will have sort of a rough surface to grip. Sand it down an it becomes very sleek. Well, with petg at least. Drilling the plastic could easily ruin the whole part.

I added example picture of the slug I just made. Since it is for testing purposes only, I also added text to the side to show the pitch and on the top of the slug there is a scale that shows the oval angle. It has a thumbhole pitched .150" forward with oval angle of 45* :)

Remember that in order for the slug to be legal to use in bowling balls, it has to have a hole all the way to the bottom of the slug. It is not allowed for bowling ball to have a not solid area inside of the ball. That is why all the switchgrip slugs are drilled as deep that there is a hole through the bottom.

I can also help to customize slugs if needed. I use Solidworks for modelling. Send me a PM if there is any problems.