Hi Jean, thanks for your comments. I certainly would not expect Aquatica to endorse this madness, and clearly this is done entirely at my own risk!
However I am interested whether you agree that the 4" hemispheric glass port should have a theoretical higher pressure tolerance than the larger acrylic domes, which I believe have been known to pancake or invert at depth?
With the lens shade shaved, this port is optically acceptably matched with the Sigma 15mm?
Well the 4'' does have a small radius which is in this case does work for you, on the other hand, I have yet to see an acrylic dome invert itself, be it a 6'' or an 8" !!!
Such a small dome on a full frame camera will be a tough one, typically small dome are designed for shooting up close, with a cropped sensor (shorter focal length and smaller image circle) at the minimum distance with the smallest of aperture, you will be shooting a wreck at some distance on a full frame at close to, if not, full open aperture. you will be seriously out of the comfort zone of this dome design Harry.