To drill a Vise IT, you really need the Ferrari jig ball, and the proper sized adapters for it.
The adapter has a notch that fits into the groove of the slug which eliminates any rotational pressure that you get while trying to drill while installed in the ball.
If you install the IT un-drilled slug, then try to drill it, the threaded area at the bottom is likely to break.
Also the Ferrari jig allows for much easier drilling of oval thumbholes.
I also used the Ferrari jig to drill oval thumbholes in non-IT slugs that were going to be glued into the ball.
If you have any kind of quarter scale, you can do the angles with a a few extra lines, and a little bit of math.
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