Thank you for the image and description. I think this is a good solution. If you still want to use a shade for topside shots you can as well use some generic screw-in shade.
Update: ikelite started making an extension that is likely the perfect distance (between the best two above) so it shouldn't show the port at 10min. Going to check it out this weekend or next.
Having the ability to zoom from 11(/10)-20mm would be great for fish portraits. Have you already made the test with the new extension?