A few crude methods mentioned here, as well as a few impractical ones.
The original question was about a square (or rectangular) hole in a plastic BOX. Now, these things are usually quite small and often the hole is required in the side wall, not in the removable lid.
So, saws that hold the blade at both ends are out. So are hand nibblers since its handle cannot be moved inside the box and doing the nibbling the other way round means marking the box on the inside - not easy either if one wants accuracy.
What I have been using for years, and very successfully BTW, is what's called an 'abrafile' It has a very coarse abrasive coating on a non hardened shaft so the file can actually be bent somewhat. Mine has a diameter of 1/8" which means I drill a 1/8" hole at each corner of the cutout and saw file the bit in between out.
This works very fast in plastic and moderately fast in diecast boxes.
I have used the same abrafile for years and its done hundreds of cutouts by now. If one uses this tool carefully, just a light finishing trim with an ordinary file is required at the corners, to get them square.
Klaus