One of the members on this forum wrote abou tsuccessfully using cheap  polycotton sheeting  instead of glass matting in a build.    
      Here's a link from instructables (via Ducksworks Yahoo group site ) which I  found interesting and informative .  
               
http://www.instructables.com/id/Poor-Mans-Fiberglass-make-nearly-anything-weatherp/?ALLSTEPS