The exact parts used for the focusing helicoid intended for old Fisheye Nikkors:
1. A standard K-3 ring
2. A 12-31mm M42 helicoid (eBay, many similar, but select the narrow range as the helicoid needs to compress to a maximum to give infinity focus)
3. M42-52 step-up ring (a flat version, preferably)
4. A Petri M42 adapter + stock factory Z bayonet + epoxy glue to make a flat M42-Z adapter. Probably something similar should be commercially available.
I already had made several of the part (4) so spent most time rummaging through my bins to find parts (1)-(3)