Bud, It is time to replace the large windows on our W42.  They were last replaced 10 years ago with 20mm (3/4") tinted polycarbonate.  I have not been successful in locating tinted polycarbonate in that thickness, nor have I been able to find cast acrylic or laminated safety glass in that thickness.  Do you have any suggestions?  How about shimming the interior ring and going with 1/2" acrylic which is readily available?  Hope all is well.  Thanks.