I made this cup holder out of a 4 inch ID PVC pipe. It was mainly to be used at an old baseball field that doesn't have cup holders built in. It was designed to work on the railing where my parents have season tickets to, but it works well on the back of an empty chair too.
I built this on estimated measurements, if there is interest. I will post a full Instructable of the next one i make.
I would like to thank Thinkenstein in http://www.instructables.com/id/PVC-Its-Great-for-Inventions/
for the idea.