how to create square holes in a box?

Obviously the best way to make the holes for the buttons in a box mod is with the proper sized drill bit and a drill press. But what about the square or rectangular holes needed for the OLED screens on a DNA 30 or sx 350?
So far I am doing it the same way as the round holes, slightly smaller drill bit, getting almost the right size opening and then finishing it with a set of small files.
Its not so bad but it takes a very long time with the files to make sure I keep the lines straight, don't over cut and still get the metal finish to a decent standard.
Is there a better way short of having machine shop do it for me? I mean if I am going to go that far I might as well have them make the entire box to my exact specs instead of modifying a Hammond, right?

A dremel with a cutting disc could reduce the amount of filing. One little slip though.....

A production facility would have a custom square punch on hand for that shape.

So a Dremel is your best bet. Just be very careful and take your time!

I find for the home shop the easiest way to get quick and professional results is to go just inside the 4 corners with a very small drill bit then dremel cutoff wheel (the thin type) between them and finally, hand file the final 1/16" with jewelers files. Takes about 10 minutes and gives very consistent results. Be sure to use a very sharp center punch to locate the 4 corner drill bit holes before drilling for accuracy. If the corner drilled holes are accurate, the rest is trivial.

Square bullets

But the drill the corners then dremel/file the rest is the easiest

Chisel mortiser

Make sure you run the Dremel at high rpm with those thin disks , i prefer between 30 and 35,000 rpm and make sure you have eye protection and a sturdy comfortable grip .