I realized that the longest M3 screws I had weren't long enough. This is used to couple two screws from either side together with two nuts integrated inside. There are two versions: slotless and slotted. The coupler must be printed on its side in order to resist tensile stress. I have not tested the load limit of either versions.
In order to print the slotless version, I printed the with supports and paused the print around height 6 mm to insert the nuts (see image below).
The slotted version can be printed without supports.
The author marked this model as their own original creation.