We made a kind of joule thief flashlights, based on a construction derived from my cyborg zombies http://www.instructables.com/id/Cyborg-zombie-feeding-on-quotdeadquot-batterie/ The battery holders were made of cardboard, but that proved to ...
The basis of each gondola is formed by a piece of steel wire allowing for it to run “in” the wheels. It starts with a short end (about 1,5 cm) to be attached in parallel to the rope. Then follows a bend bringing it outside the wheel’s rim. Some more bends are made as a counterweight. The latter is not necessary if the gondola itself forms a counterweight.
Take care none of the inwards bends measure further than half of the wheel diameter, as otherwise the gondolas will hit when passing each other.
In the picture you see how the small “connection” piece is slightly bent "out of the plane" to bring equilibrium in tha direction too.
A small piece of tape put on the steel wire end is rolled tightly over the steel wire and looped rope together. Check how it fits in the pulley in the last picture. One kid operating the motor till the desired spot of attachment passes and stopping it so another can attach his or her gondola proved to be a great little game of collaboration.