If you can't find a suitable supplier who does uni connector bores (/can do them yourself), it's also an option to go with external corner brackets for everything instead.
Those are generally easier to find, and more flexible in slot dimension requirements etc.
Of course, like uni connectors, they have their own considerations.
The main thing is to find a suitable profile series, and to stick with that as much as possible.
(For example: 40-series metric, i-type slot-8).
Makes your life a lot easier.