Ok, after a couple of days of testing and iterating on the design, I think we are close to a winner!
The latest version fits better, and has a been reinforced on the "bottom" part which I think is where many of these things break. You can see the size difference here:
This one fits perfectly:
The enlarged/reinforced end still clears the rest of the mechanism with over a 1mm of clearance:
I've placed an order with shapeways.com for 5 units made in nylon. This comes out at $8 to $9 each. I'll let you know how the shapeways units turn out.
Cheers!
Ian.