I'd recommend printing my newest version over this one: http://www.thingiverse.com/thing:140048
I got a ton of requests for this at Maker Faire NYC, so here you go.
I really liked emmetts iris box but was having trouble putting it together and the pins kept breaking. Using his as an inspiration, I built this from scratch to use flat head screws. This version will work very well after some tweaking.
I have included a small and large version. There is also a shorter version of the small base to make it as compact as possible. One disadvantage of this design is that it can't be scaled up or down unless you figure out what size screw will ride in the track properly.
I have been using 5% fill and 1 shell with good results.
You will need #4 x 1/2 flat head metal screws for small version and #6 for the large version. They must be flat heads or it probably won't work.
After printing, layout the iris blades and put a small piece of tape on the top to hold them all together.
Put the iris leaves on the pins and lay the gear ring on top.
Put screws in through the bottom and get them started. Screw them all in until they just stop.
REMOVE THE TAPE! I constantly forget and can't figure out why the iris won't work.
Now adjust all the screws in/out until the iris moves smoothly. This can be a bit of a pain. I find that after screwing in all the way, backing them off 1/4 - 1/2 turn is a good start. Usually 'all the way' is further than you think. There shouldn't be much play between the ring and iris leaves.
Depending on the quality of your print you may have to clean up the edges of the blades so they close properly.