Borbin the 🐱

The support ring of the 7Artisans Fisheye for Nikon Z mount is 55.04mm, while the Nikon Z mount itself is 55mm.
Tight fit. You can barely screw that thing on.

The Nikkor 16-50mm is 54.91mm.

Even a cheap F to Z mount adapter is within tolerances.

To fix this, first unscrew the lens mount.

The support ring needed to be reduced to 54.60mm. The mount is not perfectly round and a first trial to 54.80mm was still too tight.
Chinese tolerances of the overall mount, I guess.

If you don't have access to a Lathe, you can use a fine file and make several rounds to take off the 0.2mm. Take your time and use tape to protect the ring.
Note: The mount is positioned with the red dot next to the Aperture print.

Done, Lens meets Camera.
Now it fits mirrorless.

Update:

The lens is sharp only on one side.
Crop from the top part of the test picture. This is with f/5.6:

Camera rotated 180°:

Stopping down to f/8 improves things and of course its getting better with f/16.
Pay for a f/2.8 lens and use it like a pinhole camera.

Another creative feature is the remarkable colored lens flare when you point towards the sun with f/8-f/16. You get that for free when doing a 360°×180° panoramic image on a sunny day.

Update of the Update:

I contacted 7artisans through both their main site and store but received no response.

The issue was resolved by switching to a TTArtisan 7.5mm f/2.0, which matches the sharpness of my 7artisans copy even at f/2.0. The support ring measures a perfect 54.90 mm in diameter and mounts smoothly.

With that, my chapter with 7artisans ends here.