Here's the eye iris I'm using. They measure 37mm OD x 5mm thick- perfect size! The iris apertures are available from China via eBay but you can find them in the U.S, from companies like OptoSigma and Thorlabs. The trick is finding one with the correct dimensions and we just lucked out with an eBay find as they were a lot less expensive than sourcing stateside.
Full open to full close requires rotating the pin lever 90 degrees. Because they are identical, in order to operate them properly the right and left servos need to rotate in opposite directions. The way I've designed it the pin lever is fixed and the iris body rotates- it's housed in a machined cup that rotates in the bearing mount.
I machined the eye cups from solid Aluminum round bar stock. Each iris is held in place with three 6-32 set screws. The cup has a shoulder that sits against the 1/2" bearing mount and it's held in the bearing with a 1/2" Actobotics clamp. A small brass arm is attached to each clamp and a short ball link rod allows the servo to rotate the iris cup.
It's a perfect fit in the head casting… no room to spare!
These two pics show the range of iris movement from full open to full closed.