Gather the PVC, the gasket assembly, the loose-fitting piece of acrylic glass, and some glue.
Turn the gasket assembly upside down and center the PVC upside down on top of that. Put a thin layer of glue around the inside edge of the sheet metal ring. This glue is for the acrylic glass not the PVC.
Center the PVC on the sheet metal ring and then put the piece of acrylic glass inside the PVC onto the glue. Make sure everything is centered. Once the glue starts to set, take the PVC off so it doesn't accidentally get glued in place. Carefully put a weight on the acrylic to hold it in place until it dries.