This is the most complex replacement part I ever made and it would be probably cheaper and much faster to just buy whole new garden chair. It took 11 iterations to get satisfying result, but now it serves 3 years without problems.

As this part is quite stressed (which is reason why original part got broken) I decided to use carbon filled nylon and steel screws to strenghten critical sections. Due to relatively high thermal expansion of Nylon, I used relief pattern on more narrow surfaces to reduce warping tension (controlled by antiwarp variable in the source code).

For printing, supports are required. For final assembly, 2× M6×55 screws + nuts and 2× M2×12 screw with flushed head.

Credits

Metric thread library by Dan Kirshner: https://dkprojects.net/openscad-threads/