I've been working on the Nessie project. I've changed the design of the sleeve so that the main slider sits in a U shaped channel rather than being using a tube. This is mainly as a way of keeping the slider tube in position. I was concerned that the old design which was spaced away from the box by a narrow rectangular tube could easily move up and down. I've left the top of the channel open as the rack is held in place by the two wheels. This leaves plenty of space for the head and tail mech

The slide fits over the top of the rotating pin to complete the scotch yoke front and centre. The two hump wheels then fit over the slide completing the double rack and pinion and holding the slide into place. 

From the front you can see the axles supporting the wheels and the handle which drives the scotch yoke.

Turn the handle and the humps rock back and forth, the head and tail will move up and down.

Head and tail mech next. Not sure whether to use a bell crank or a cam surface on the top of the slide. I'll probably end up trying both and see which works best.