Baader makes a Nikon Z-mount wide T-ring. #2408335
It comes standard with a M42x0.75 adapter, but it is interchangeable with other adapters and you'll need the S52 to M48 adapter #2958552 to make it work for M48.
I have Canon RF mount and found that they were the only ones who offered native mount like this for the new mirrorless models that could be made to work with M48.
Agena Astro sells them. The native mount is $32, but the tiny S52 to M48 ring is additional $50. I wish they were included.
Here is a trick. The Baader #2408317 adapter, for the Sony E/NEX mount cameras includes the S52 to M48x0.75 adapter #2958552 as standard instead of M42 ring, but includes a M42-to-M48 adapter ring. You can buy that for $39 from Agena. Even if you don't need the Sony adapter, you can use the S52 to M48 adapter and save some cash and also have a spare M42-to-M48 adapter ring. That is what I did.
These rings are very thin, with only a 8.3 mm of optical length, which can be helpful in some situations where you want to minimize backfocus. The "typical" T-ring is nominally 11.0 mm, but I've measured mine standard Canon EF M48 ring from William Optics to be 10.30 mm. Remember, if you add a filter in-line, it changes the backfocus slightly. Rule of thumb is add one-third of filter thickness to backfocus. Typical filter thickness if 2 mm, so add about 0.7mm thickness for backfocus calculations, which is why I think that the T-ring adapter are either 10.3 or 8.3 mm. Canon EF flange focal distance is 44 mm, so 44+10.3+0.7 = 55 mm, so you wouldn't need any additional adapters. With my Canon RF mount, the RF flange focal distance is 20 mm, so 20 + 8.3 + 0.7 = 29 mm, so I need an additional 26.0 mm of adpters to make up the backspacing. Could add a filter slider and a 5.0mm adapter for example. For your Nikon Z-mount, the Z-mount flange focal distance is 16.0 mm, so add the 8.3 mm of this adapter and you'll have to make up the 55 mm backfocus difference with additional spacers. So 55-16-8.3-.7 = 30 mm and that is how much spacing you'll need between the Baader T-ring adapter on Nikon Z and the Raptor M48 threads if you use a filter in the optical train.
Edit: OK, just noticed that William Optics now has Canon RF-mount 48mm adapter as well. My guess is optical path would be 34.30 mm, but unsure.
Edited by drhollen, 18 November 2020 - 03:00 PM.