Hoeken's Coupling

This is a shaft coupler / adaptor. It uses a split-coupling design that allows it to clamp onto the shaft. Even though there are horizontal overhangs, they don't interfere with the operation of the coupling. If they bug you, they are easy to cut out.

The openscad design has the following parameters:

shaft 1 diameter (smaller of the 2 shafts)
shaft 2 diameter (larger of the 2 shafts)
bolt size (M3 = 3)

there are more derived parameters within the module such as wall thickness, gap, diameter, etc which aren't very useful to call from outside the module.