Print the parts. Use a solid profile for strength. Clean up. Run a 3mm drill through the clamping holes.

The clamps will have good clamping force, but limited travel. The part should press onto the shafts with some reasonable force. If they are loose on the shafts, please make the shaft hole sizes slightly smaller and re-print.

Take the printed spider, and use it as a pattern to cut a spider out of rubber. The thickness (Z) of the rubber is pretty important. Too thin and the coupler halves will touch. Too thick and it will get wobbly. The shape is also critical as far as symmetry. If it is not symmetrical, the shafts will wobble. The absolute size if the X & Y can be a little large, it will just "pre-load" the coupler a bit, and assure no lash.

A softer rubber (perhaps cut from a sheet of neoprene wet suit material) will give more cushioning. A harder rubber will make a stiffer coupling. I have considered using a slice from a rubber eraser. Maybe slice up a rubber bumper?

If you build this, please feed back what you use and how it works for you -- thanks!