Clamp it in the vise with the opening toward you and maybe some
cardboard between the box and the jaws. If you are worried about
clamping pressure put a block of wood in the box so you are clamping
only the bottom and a parallel or such behind it if it has molded feet.
Mill it with a 1/8" bit at the highest speed. I'd mill it square and
file the longer side. The mounting holes can be milled out from the
opening or drilled separately.

Size the hole for the male connector regardless of which one goes on
the box.

jw