Helical flexible couplers are just crying out to be 3D printed. But to avoid steep overhangs with a standard coupling design, you need a really high helix angle, which makes it fairly stiff in bending (bad) and not-so-stiff in torsion (also bad). This design uses a sort of conical/helical slot with a low helix angle that eliminates the steep overhang.
Additionally, is has a small diameter solid central core which eliminates one of the unfathomable design flaws in many 3D printers. A lot of 3D printers have the weight of the Z-carriage directly supported by the helical couplers, which are essentially large springs. Small compressive deflections of the couplers will show up as raggedy print lines. The solid core of this coupler prevents any compression while still being flexible in bending.
This coupler has a 5mm bore in one end and an 8mm bore in the other. I've included 2 different versions - one has slots for M3 nuts, the other has plain holes that you can simply thread an M3 set screw into directly.
Use 4 perimeter lines for printing for extra strength