Another version of the mechanism...

I've been struggling with the previous mechanism. Due to a lack of precision tools I am struggling with measurements being ever so slightly off which is causing friction and other problems, meaning that parts are not moving smoothly. Onto my next design.

This is a rough mock-up on a scrap piece of wood. Some parts are nailed down and the guitar wire is held in place on the main block (which will hopefully represent the head and legs) with masking tape, just to get an idea of how it will work. 

It's really simple, space efficient with plenty of room for the guitar wire to move but rigid enough to hold position and move the block. I wish I'd though of this sooner. Even with all the parts cut roughly and not measured properly or straight, it still works very well. The only problem I have with it at the moment is that the guitar wire makes a scratching sound as it rubs against the edge of the inner tube. Maybe I could somehow smooth things out.

The openings in the turtle's shell for the legs have a thick enough wall which could act as runners. The other idea I had (which might not be needed) would be to have a cavity on the underside of the legs and a pin which the leg could sit on, meaning the leg would travel along the pin as it guides it back and forth (hope that makes sense).

Time to try again.