To get the concept out of my head and into a form I could play with, I modeled the rig in Sketchup.
The build was straightforward thereafter. It was simply a case of drilling the right holes and bolting it all together.
Having built it the only issue I had was the belt slipping. In order to keep the timing belt teeth engaged I added a roller to lock the belt against the pulley.
And the first model is complete!
I have further improvements to make - primarily boxing up the Pi, batteries and cables, and will update this site as I make changes.