Others have made them, and I have printed them... and the ones I have printed have always broken, in the middle of a print.
I think this design will have better success as it eliminates the failure points all the others I have used had. It uses actual metal nuts in the provided openings that can be placed after printing, and either 3 or 4mm set screws. I also provided couplers that accept either 2 or 4 nuts.
As always I have included source files for easy customization in the form of STEP (.stp) files. STEP files can be opened in a variety of different CAD packages, such as AutoDesk's FREE to use Fusion 360.
The ID's have been slightly oversized because my printer tends to print ID's a bit small. This is intended to be used with a 5mm stepper motor's drive shaft and an 8mm lead screw. The actual ID's of the design file are 5.3mm and 8.3mm respectively.
As long as your printing one, print two. That way if one fails, you have a spare to print more of these couplers!
Print, slip some nuts in, align the nuts with the holes and begin threading in a set screw. Couple your motor's 5mm drive shaft to your 8mm lead screw and snug up the bolts.
Most aluminum couplers come with either 3mm or 4mm set screws. I had both, so I conveniently made designs for both.