I simply designed this z-axis extension vor my Anet A8 printer because the Z-Axis threaded rod was a bit to short after i installed this self centering coupler.
If the straight rod sits too lose on your printer check out this:
For easy assembly i made the:
Top diameter = 7,8 mm
Diameter for rod = 8,2 mm
Print with 100% infill.
If it is a bit too lose because your printer has a high accuracy try out some teflon tape or cut a stripe of paper and put it between.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/8dgVjpkzfnB