Just a quick thought that may or may not work. Would it be possible for him to fiberglass up a tank and incorporate the glass viewing window while laying down the layers of fiberglass? I've never done fiberglass so this may just be an out there idea, but when thinking about it this would make the viewing window an actual part of the tank that doesn't need a special seal.
Like I said, I have never done anything with fiberglass so I'm not sure how well this method would seal up the viewing window.