Could the lens be supported by a mount in the tube, rather than a cage attached to/around the camera?
“Good enough” depends on your requirements. A camera with its input at the centre of a dome will have a less distorted image than one looking through a flat window, but it’s possible the reduced distortion at the edges/corners isn’t worth the extra development/expense for you. I’m hoping to make a clear guide and possibly a calculator for that at some point in future, but in the meantime you may be interested in these web pages:
- Optics of Dome Ports
- Planar Interfaces and Apparent Positions
- https://www.deepsea.com/knowledgebase/underwater-imaging/dome-view-ports/
It’s also of course possible to try out a flat end-cap and see if the results are reasonable or if a dome is required.
Our 8" enclosures are rated to 40m depth, so 20m shouldn’t be an issue 
Hmm, it is definitely an old application, but Windows is usually pretty good at being able to run old stuff. Is there a particular error you’re getting, which isn’t covered by the troubleshooting manual in the Documents and Downloads section?
If there is you can perhaps try one of the approaches suggested here, but I’d recommend checking the operating and troubleshooting manuals first