Is there a good Arduino enclosure for that works with the Olimex MIDI shield
Your best bet is to 3D print the enclosure.

There are many places that will 3D print your enclosure for you and most will mail it to you as well. I would recommend Shapeways as they are fairly reasonable in price and ship worldwide.

If you are a handy DIY person you could use SketchUp or Tinkercad to design your enclosure. Also there are many Arduino enclosures on Thingiverse that you could use as a starting point.

Alternatively there are often places that will design (and then 3D print) the enclosure for you, you just have to supply your hardware. There are often a few places like this in each city.