SailRite sells Plastipane, a clear vinyl material for windows in fabric.

They have it in .020, .030 and .040. Only the .020 is in roll while the
heavier is in sheet form..

They also sell Strataglass which has excellent clarity and scratch
resistance. It is available in .040 sheets.

There is also a 'sidebar' with good information on selecting your window
material.

Steve
s/v Good Intentions