My understanding, (dare I say it? from Don Casey's book) is that lexan is
superior in every regard except price. He particularly likes Lexan's
resistance to internal crazing. He goes on to outline a project to cut out
the top of a fiberglass hatch and replace it with smoked lexan/polycarbonate
to allow more light down below. Like everything else in that book, it seems
quite simple the way he explains it.
I will probably do a similar project for my forward hatch, but I will likely
just build a new frame altogether and make it from varnished mahogany. I
think another splash of brightwork (or bluework, if you're Tim :) would look
nice up on the foredeck. I think I'll also make an acrylic canvas cover for
it to keep varnish maintenance to a minimum and to help prevent the points
brought up about UV and wood rot.