Hictop prusa I3 clone - broken z axis right bracket bearing
That "plastic coupler thingy" is actually a linear bearing, and it consists of a metal shell, and the metal balls roll between the smooth rod and this shell. The plastic part has a channel that guides the balls in a circle.

It would be best to replace the entire bearing (metal part+balls+plastic liner). It's probably either an LM8UU (most likely) or LM8LUU bearing.

3D Printing a new part will not give good results.