Here's a spider coupler for coupling two axes. Includes a cutout for a nut and a screw for attachment.
It's fully parametric, 16 parameters can be edited straight in your web browser.
Try the online version at:
Scroll down to Examples and choose Parametric Axis Coupler
Or download the .jscad file and use the OpenJsCad parser:
A modern browser is required (for WebGL support).
For each side of the coupler the axis diameter, inner depth and outer size can be configured, as well as the size of the tightening srew and nut. The 'spider tolerance' parameter slightly reduces the spider size, allowing the thing to absorb some wobble.