Thanks Dan, My lenses will be 6" in
diameter and 1 1/2" thick and my flanges are pretty beefy with 3/4"
thick wall and 3/4" seat. I think with care hopfully I won't see any
significant distortion after welding. If I end up with some variation
with how the lens is flush with the top of the flange I could vary the
thickness of the gasket material on the seat to bring the lens up or
down. The lens needs to be flush with the top of the flange because
the O ring has to get compressed by the outside retaining ring. I
will have a V notch on the edge of the lens. The gasket on the seat
also will act to seal the water out.