The Nessie model will have two humps which will be rocked back and forth using a rack and pinion.

This is the latest draft of the hump part. The pinion teeth run the length of the bottom quarter of the hump. These will engage with teeth on top of the main slide, driven it turn by the scotch yoke.

I had originally planned to add a cross piece in the centre of the wheel. You can see the tabs where it was to be glued into place. In the end the piece was unnecessary so I've left it out. 

Gluing the rim of the wheel into place is quite tricky, especially the last stage where I just can't get my fingers in. To get round this problem I'm going to try using two ring donut shaped pieces to make the wheel. Once the wheel is complete I'll add the white axle side piece. That should make construction a lot easier.