Best port for canon 8-15

Looking for reccomendations for a dome port for cannon 8-15 fisheye on a nauticam housing and sony a7r3.

I'm considering the zen 100 dp as well as wider 7-8 in domes. 

Thanks

If you want to go glass you have three options that I am aware

Zen 100

Nauticam 140

Nauticam 230

For the type of shots I use the fisheye for the 230 is not practical and as it is heavy I would prefer an acrylic one for splits so I have chosen the Nauticam 140 on the basis that this lens is a fisheye zoom so a larger radius will help when the lens is at 15mm on my MFT set up.

However on a full frame this may not be relevant as you have the bubble look and the diagonal fisheye 

How much of a difference is there in image quality when shooting a fisheye in a 140mm vs 230mm dome?

Probably very little, fisheye and rectilinear optics are different and the fisheye has no trouble taking a 180deg curved virtual image and placing it on a flat sensor.    - assuming that both have the entrance pupil placed at the centre of curvature of the dome.

Big domes are used for rectilinear lenses as the radius of the virtual image is larger and you need don't need to stop down as much to bring both the corners and centre into focus.  This is not as necessary for fisheye lenses

Article by Dr Mustard

https://wetpixel.com/articles/review-nauticam-140mm-dome-port-by-alex-mustard

He says 140 mm is the minimum acceptable dimension for a full frame camera and 100 is only good for cropped sensor.

230mm will be better in corners but is much more bulky