A few ideas popped up:
- use a case from an old pocket watch or stop watch;
- build a machined-aluminum (CNCed) case from scratch (this would be an expensive venture), similar to that of the Nixie watch;
- stich a tennis-cuff-like "soft case", with a transparent film material on top;
- print a plastic case with a 3D printer (e.g. reprap);
- screw together a few layers of laser-cut acrylic.
Here are some links I found after googling around:
plans for laser-cut case for Makerbot watch;
- plans for 3D printed "parametric Makerbot watch case";
- posting on Make with photo of printed 3D case;
- another 3D printed case for Makerbot watch;
- (added Dec 5, 2010) other set of plans for a laser-cut case for the Makerbot watch, on thingiverse.