A simple aperture from a camera. Still needs testing; I don't have a 3D Printer yet.

Updated with better clearances.

3D Print the Inside Ring, Outside Ring, Top and six of the pins.

Print the DWG out onto a piece of card or similar (don't use anything thicker than 1mm) and cut the parts out. You could laser cut them if you have access to a laser cutter.

Assemble and your done!