I have always felt that if you cut the POLY firewall with a shoulder (rabit?) that not enough material is left for the screws holding the firewall to the PVC. If you put in a plain POLY firewall and then cutout four POLY right angle triangles with two equal sides about 1 inch, they can be located at the corners of the firewall so they overlap the PVC and thus serve as a shoulder at the corners. One screw in each should do it. Might work fine with just two triangles in diagonally opposite corners. Comments?