Tapered Washer for 3 Inch PVC

I needed to make a large humidifier for a mushroom fruiting tent. Using a floating pond fogger in an all black tub, I mounted an AC Infinity 120mm fan to force the fog out, and cut a 3 inch hole for the PVC. The PVC pipe tapers so when you screw the parts together it doesn't mate flush. 

Using two washers, you can increase the amount of purchase that grips the lid much better and allows it to seal up well with out any adhesive.