Brendon Collecutt, who developed the STEMFIE OpenSCAD library, STEMFIE Roberval balance scales, and a STEMFIE sailcar, recently published an enclosure for N20 DC motors.
You can download the files from this link on Brendon's Thingiverse projects.
STEMFIE N20 Motor Enclosure
This toy is unsuitable for children under three years due to choking hazards! 
