If you can remove all the "internals" from the monotribe box, it should be easy. If not, and you have a clear space of about 1-1/2 inches (no components or wires) behind the surface a stepdrill as rcrowley suggested is still a good way to go. Obviously, be sure that the drill cannot touch any parts.
If there are parts very near the hole location, you can drill a small hole (1/4 inch using a short drill bit or a "drill-stop", and then switch to a chassis punch
to enlarge the hole for the MIDI connector. MIDI connectors require a 0.625 (5/8 inch) hole.
Remember, you do need some space behind the hole for the MIDI connector body and wires.