It was pretty simple, actually. I cut a sheet metal "ring" and used a die grinder with a carbide bit to put in the 3 notches where it bolts up. I measured the ID (inside diameter) of the "ring" (I think it was 6.25 inches) and multiplied it by "pi" (3.141592654) to get the inside circumference of the "ring" (19.635 inches). I then cut a 2 inch by 19.6 inch piece of sheet metal and rolled it into a circle to fit inside the "ring" and then welded it all up.
The only problem was the 3 notches. I had the "ring" upside down when I welded in the vertical piece and it didn't line up. That's why you see some extra notches in there.
I put a lot of epoxy around the "ring" and jammed it up in there and put the nuts on until it cured.
I haven't poured any water in the vents yet to see if it leaks, though. But, with as much epoxy as I put on there, I doubt it will leak.