Best way to cut mounting squares out of plastic project boxes?
If the plastic isn't too thick, then you could try this:


Cover the surface of the plastic with masking tape, so you don't scratch it.
Mark out the rectangle you want to cut out.
Using a very sharp knife, score the edges of the rectangle against the side of a metal ruler.
Cut deeper and deeper grooves with the knife, until you can eventually push out the rectangle.
File any rough edges, and remove the tape.