Update 7/2/2025: Somehow I printed two for the same side, so I just installed the 2nd on the back post (other side). The design has been slightly changed to allow cups with handles to set in the holder from either side. The opening for the handle on the cupholder has also been narrowed down to about 19mm wide at the base (it opens up as it gets higher on the holder to about 33mm). I will probably print one more cupholder to test the latest changes, but at least I can use all of them (with three on the swing).
The models have been uploaded to MakerWorld, and you can find them here:
https://makerworld.com/en/models/1573771-outdoor-patio-swing-chair-rocker-cupholder