A cup holder designed for the IKEA POÄNG which should work on similar chairs as well. Also fits nicely on the footrest.
Note: the default diameter is 6 cm, which is just enough for my longdrink glasses, but too small for wider glasses or mugs. It can be easily tweaked to match other sizes: I've included the Fusion 360 file and all the dimensions are under Modify - Change Parameters.
If you don't want to mess with Fusion 360, leave a comment with the desired diameter and I'll create an STL for it!
I designed it as two parts to minimize the amount of supports needed. Although definitely not required, I did weld them together afterwards using a 3D pen. CA glue did not work for me due to the rough surface from the supports.