I made an iris diaphragm out of cardboard, and it works. I am now designing this to be at least 3' in diameter. Why so small? Because our Shop Bot is only 4' x 4', and I do not want to have more than five unique parts. The above cardboard prototype is three unique parts.
fruit iris diaphragm
I made a small acrylic iris diaphragm, and it turned out looking like a strange fruit, which was completely unintentional. Anyhow, I thought I better make a prototype out of rigid material with a handle. The green stem-looking thing is the handle to open and close the iris. It could be a mutant watermelon/cherry/tomato that tries to bite your hand off--that would be a great final project!