Reading University has a 3D printer, which we planned to use for this project. It uses ABS, which is durable and despite only being available in one colour,c an be relatively easily painted if necessary.
The printer is compatible with Solidworks (or anything else that produces .STL files), which allows for precisely-made parts to be produced to scale.
The case, littleBits connectors, and the three ‘top’ designs have been printed (still learning to use the camera!):
This is the hex-top. It has had to be extended upwards to accomodate the internal parts.
The current design makes them relatively easy to change, but this does mean that the tops could be removed by a user. Future versions will require bayonet lugs or some similar mechanism to hold the top on.
The littleBits connectors are generally the right size, but the small prong and socket is too fragile in 3D-printed ABS. They are also slightly the wrong size and do not fit cleanly. It has already been noted that they will need holes for magnets and electrical connections, so future versions will incorporate these changes.