For "a few" (I see no indication how many you are making - 1 - 100,
1000, 10,000....), I'd probably just lay out the holes with a scribe and
punch them (did a lot of one-off electronics enclosures in my lab work),
then drill on the punch mark. This is more than adequately precise for
fan mounting (BTDT) with a careful hand and a good drill press. If
you're talking hundreds, jigs are probably in order. If you get to
really high volume, some sort of punch press to stamp them out might be
justified, or (for more flexibility if the parts ever change much) a CNC
mill with a larger table.

This is definitely small volume - one for now, a handful at a time
later. The surface is a little rough for good layout, and I still
cringe over the thought of my previous work on these panels.



Mounting it on the mill you have sounds like a pain, and overly precise
for the job at hand (ie, takes much more time fussing than is needed to
do the job adequately, thus, wastes money).

One caveat: mistakes are needlessly expensive in this case, because the
plates are "medical equipment". Some degree of fussing is appropriate.


Only reason to alter that
thinking is if this is a one-off hobby project and you want to spend
time playing with your mill...

Fair enough, but you should have been around for the first set of these
things =:0 It was enough trouble that I plan on some amount of
overkill. A jig for the punch is probably a nice compromise.

Thanks!

Bill