Let me see if I can find another post on this topic. Several users have done different things such as I believe a large McDonalds straw, @bjcowles I believe mentioned this. I think a tube of the correct dimension would be what you want.
[Edited to include a link to a few examples in another thread]
https://community.hubitat.com/t/ball-and-socket-for-iris-motion/3036/16
https://community.hubitat.com/t/limit-iris-v2-detection-area-ideas/34551/4
