This is the plastic square which is at the end of the bar which connects to the bicycle.

These 2 parts are initially glued at the end of the bar but mine came lose after a while and I ended up losing one of them. These replacement should work just ad the original. I have printed them out of PLA, we will see how long they last.

Print the 2 parts and glue them together just like the original parts.

EDIT: The fit on the bicycle side was a bit tight so I made a new set with 16.8 mm for the inner square. The files for this version are the 168 STL files.