Remove batteries and four screws under the rubber feet at back bottom panel. these pictures show the easy way to do it. The metal dust of enclosure when you dig holes may make short circuit or something. So you should better to remove all the components inside of the monotribe. but re-build it correctly may be not easy.
any way lets get find the connectors that we want, first.back bottom panel is connected to power board of the monotoribe inside. you can also remove it.

Double line connector is MIDI-IN. And triple line connector is MIDI-OUT. before do something on to your enclosure, test it if it works fine or not.

Download the drawings from next link

print out it And cut it out just the same as the picture above and make it as the template to digging holes on the enclosure.

As I wrote before, you should better to remove all the components inside of the enclosure to avoid the short circuit by metal dust. you should better to use vacuum cleaner when you use the drill or so.
there is small tips to digging hole at correct position. Don’t use 6mm drill first, you should better use more thin size, for example 3mm or so.
and change the size of drill and spread it as you need.

Don’t forget to connect the connect power board and back the bottom panel before put back it and re-screw.