Lexan is a brand name for a particular patented polycarbonate material. It's production was take over by a company called Sabic in 2007. While it is prone to easier scratching, it can be polished back to full clarity. There is a seller on Ebay that offers it in any thickness up to 1/2" and any size you want. I just received my 2 pieces of tinted 26" x 26" I paid $100 shipped as opposed to paying $150 for a 4' x 8' sheet and then have to drive 100 mile round trip. I would have a bunch of scrap I have no plans for now, so the premium for custom cut pieces was justified. When cleaning Lexan always use only up and down strokes, never circular. That way the reflections off scratches don't drive you crazy going in circles in front of your face..
I think the question is vague, do you mean a Lexan cover on the "roof" or the "ceiling", there's a difference. I've seen appropriate sized bubble dome skylights that would work well over a shower.
http://search.myway.com/search/AJima...pr=sbt&trs=wtt