Cup holder attaches to the chair frame on a pivot so it's level even when the chair is reclined.
The mounting bracket is clamped on the frame, and the cup holder can be easily removed to fold up the chair during storage.
Requires Qty-4 M3 threaded inserts and Qty-4 M3x8 bolts. Could easily be adapted to thread directly into the plastic instead of using the inserts.
I included the Fusion 360 models for the clamp pieces in case you want to adapt to different tube or hardware sizes.
Recommend using a material like PETG that can withstand outdoor exposure.
Supports are required for some of the overhang areas.
Make sure all the support material is cleaned up well in the pivot area. A small drop of light oil in the pivot area also helps.
Due to the pivot joint, tall and top-heavy containers like the large 40z Stanley mugs may not be stable. I could develop an alternative design to raise the pivot if there are enough requests.