> A screen over your viewing port wouldn't be too good optically.
Many years ago a fellow wrote an article for S&T that detailed his using
some clear material that he claimed was optically "just fine." He covered
the opening in his dome, or whatever it was, with this material and could
thereby observe in comfort during the winter. Soon after, some company
marketed his idea but it never really caught on. If you could find this
article it might give you some ideas. I cannot think of the man's name, but
something like "Zaluha" keeps coming up in my mind.