As - 99% of the time - rotor casings (or - ref: Progressive Ins. Co - "Rotaur") are mounted on people's instruments, I use 12-inch-long 1/16" diameter (choose your own diameter) "aircraft" bits.
I could re-sharpen them, but I buy a gob at a time, and toss them when they stop cutting as quickly.
I did the "3 vertically-aligned holes" thing decades ago, when I installed a very long-throw 5th valve (right hand) trigger on my F-tuba - to microtune 5-6-4 "low A", and also to offer me a 5-6-1-2-3-4 2nd partial (matching timbre / easier to focus) "low F".
lucky me: Those two pitches are the only ones which involve any slide-moving on that instrument...but maybe (??) that's why others wince, when I'm playing ensemble pieces with them.
Arguments can be offered up for drilling those holes a bit off-center vertically (to get them farther away from the ports), as well as putting them a bit off-center horizontally (so that a subtle paddle-touch (prior to playing a passage) PRE-equalizes the air pressure. I'll let others (who also enjoy "lacquer vs. silver" and "globby vs. non-globby mouthpieces") argue over those preferences, but (unlike those two parenthetical topics) there might (??) actually be purposes for those off-center venting strategies...(??)
It's also a good idea to take the rotor out, because the tubing knuckles (brazed in to the rotors) are not always centered on the spacing between the knuckles...particularly not (sometimes) with jimbo.
You can get them for a couple dollars less, if you buy a package of several, but here's one that can be bought at "laagge oohange - YET 'currently out of stock'...but you'll find them elsewhere.".
https://www.homedepot.com/p/Drill-Ameri ... /305284245