FXYDesign has a great tutorial that explains how to build a Lego monome case:

My goal is to make a LEGO case simple, strong, use as less bricks as possible. After I measured the templates of Monome 40h kits, I find LEGO have perfect size for monome.

The Monome circuit board size is 6?x6?. but it’s not at the center according to the button array. So 6.25? is the smallest size for it. On the Lego side, each brick is 5/16? wide, 20 bricks is perfect 6.25?.

The tutorial has lots of photos, and you can also download templates for 64-key and 128-key monomes.