Once I got a new set of drill bits this part has been very easy for me. I drill a small hole, then go back and then drill the over sized hole. I actually just put a foot on it and drill it on the floor. Nice bits are key.
The old way I did it was to use a dremel cutting disk and score an X where I want to drill the holes and do as above.
As for where to drill, use the nut trap part as your guide, short side down is best, backwards from how I originally designed it but slightly increases rigidity. Drill the holes over sized by a large margin so you can adjust the tool mounts a bit. You aren’t trying to screw into the metal just clamping to it. This is how I fix an odd tool angles when I switch tools.