Brendon Collecutt, who developed the STEMFIE OpenSCAD library and 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 not suitable for children under three years due to the choking hazard! 
