I've changed the shape of the balls and increased the length of the hangers. The centre ball is moved by a small lever on the base – can't see it on this picture! I'll do a video when it is complete. Just need to add colour now.

Thanks to Ed Eckel who pointed out that the cardboard version is the non-elastic  version whereas the metal version is an elastic collision – fancy getting it the wrong way round! The shame of it!