MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
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.