This coupling connects to a 5mm motor shaft and an 8mm threaded rod without any rubber hose in the mix.
I made this version a bit taller. Making it taller gives more area that can grip the motor shaft and the rod resulting in a much tighter fit.
I also tapered the top and bottom which makes it look nice but the actual purpose of the tapering was so that the top of the coupling would not rub against the inside of the Z motor mount.
I hope you guys like it! :)
Print the part, clean the motor shaft with rubbing alcohol and screw it all together. The top of the threaded rod and the bottom of the motor shaft should be touching or at least almost touching when tightening the coupling. Basically, get them as close to each other as possible. 25mm bolts are a bit longer than you need but I thought it looked cool. 20mm should do the trick.