Nikon 20-35mm f2.8 in Subal ND3

Does anyone have experience with this older setup? I can't find any specific reference to it.

Subal suggests a 40mm extension with the 8 inch FE port and a +2 diopter.

When assembled, the lens looks like it sits too far forward in the dome.

I am specifically wondering about stacking a 15 + 33. It looks better, but I am wondering if there is anything I am missing.

Also is the +2 diopter enough?

I'm going to try it in a pool, but looking for advice.

Thanks in advance for any input.

Ian

Based on a little optical theory, and quite a lot of experience with mid-range wide-angle lenses, I predict disappointment...

I'm feeling a rain cloud forming over my little parade...