First draft of the crank mechanism, looks promising but has a few problems. The box needed to be taller and longer. The shaft was too long and the moving platform didn't quite fit. One of the many advantages of designing on the computer is that you can easily make a few changes and reprint. So that's what I did…

Spot on!

Notice the triangular section on the front of the box. The box itself if very open and so easy to twist. The triangular section adds stiffness.

The end result works great. The moving platform could be the starting point for all sorts of different models. For some reason, when I look at it I see a person using a vacuum cleaner. Feet on the end of the box, hoover on the sliding platform. Hmm.