And here's the "lens" mounted in a 3D-printed ring, spraypainted silver to represent the aluminium ring. Not the greatest finish - I couldn't be bothered to dig out the airbrush and Alclad paint for this test, so it's just cheapass sparkly "metallic" silver paint. But it gets the idea across. The black plate is a piece of scrap dibond I had lying around. It was brushed aluminium, so I painted it semigloss Tamiya black, and cut a 95mm hole into the surface with a fake Forstner bit. Doesn't look half bad!
However, it's a problem to light. The cemented doublet lens element has a bad coating on it. When not lit it's okay, but when the light is shining through it from the back it causes an objectionable speckle to the inner surface. Urgh. You can see it here. That's a white LED behind a thin piece of red polyester photographic filter, located roughly where the actual internal red filter is located on the real lens.
Anyway. The whole thing is looking moderately convincing. There isn't as much light spread from the light as I'd like. It sort of ends up with a small dot of red light at the top of the lens, and there isn't much glow to it. But then, I haven't tried a real Nikon fisheye. Maybe they actually do that.