A simple NEMA 17 shaft 5mm to M8 (8mm ⌀) threaded rod coupler. It's printed in two parts which makes it easy to put the M8 (13mm outer ⌀) nut into place and secure with four M4 (4mm ⌀) bolts with nuts.
I added a hole for an M2 bolt (with 2 nuts) to secure the motor shaft as well, but was not needed. It all fits well so that enaugh preasure is put on the motor shaft without it.
The threaded rod will hold well if screwed in correctly, but you can add an other nut to lock the one in the coupler if you want.