I'm almost done with this co-axial crank-slider, just the assembly photographs to process.  A crank slider mechanism uses a crank, the rotating bit at the bottom, and a slider tube hinged onto the top of the box to convert rotary motion into an up and down, back and forth motion.
The co-axial part means that two sliders share the same axis to make a more interesting motion.  

In this project there are three tubes nested one inside the other. The longest tube, 6mm on a side, is fixed to the right crank, the middle tube, 7mm, is fixed to the left hand crank and moves 90° ahead of  the innermost tube.. The final tube is 8mm on a side and is hinged onto the top of the box.

This is a promising looking mechanism which I'm hoping will be useful for all sorts of paper projects. The 90° lag in the motion makes for an interesting, organic looking movement.