If possible, I've had our machinist take care of it with a few drilled holes and an end mill. I've also used a jigsaw a few times, but then it typically takes some grinding to finish getting it to the right size.
For fractional DIN size controllers if you do a lot of them Greenlee makes some nice punches. For HMI I use a handheld scroll (jig) saw. Like someone mentioned use a lot of painters tape (does not leave residue) so you do not scuff up the box face.
Also you do not burn off the paint like you do with a grinder.
I second the Makita jigsaw and Bosch jigsaw blades - both excellent. If there were many the same to do as on a production line I would go a different way - water cutter would be best. I also use a hydraulic punch for the holes. Hole saws of any kind and hand wound punches are painful. To finish off the jigsaw cutouts I use this but with a 13mm wide insert and belt.
Jigsaw is my preferred method, BUT I cut from rear side of enclosure to minimize scratches on paint. Still use many layers of masking tape, but front side doesn't have ANY scratches. My 2 Cents.
Lead time depends on manufacturer. One brand I use most often has always been under two weeks, sometimes only a few days before shipping. Another brand I only use when specified by the customer is average 6-8 weeks.
Everything is cut out before paint/powder coat. They do all that when they make the enclosure components. Like the door, when they run that thru the process in flat sheet form, that is when they do the cutouts.
It's cost effective for me. I have them put in as much as possible so we have little to no enclosure work.
We use punches for the buttons, glands and some standard sized equipment. For the HMI we use a jig saw. Works great (as long as it is not me doing it).
Now I think of it, I bet one could order custom hole punches in any size.
thanks for all the suggestions! at the present time, we're just doing one-offs and small runs. probably not enough to justify CNC laser or plasma cutting. I like the jigsaw idea. what saw blade would work best for this? i would guess it would have to be one with a very fine tooth pitch..