If you're handy with hand tools, you could make your own.
I've seen rolls of the clear and the tinted sheet vinyl at the fabric store. Not the super heavy stuff, but highly flexable and cut to the lengths you need. I cannot say if that's the long-lasting, UV-treated window material that you'd want to put on a boat, but it's cheap. I want to say that they had clear in two thicknesses, yellow, green and smoke tinted rolls.
Then use a screen frames from the hardware store, but instead of installing screen, you'd install the vinyl stuff. Standard screen spline ought to hold it.