I went a similar but different route with the cleanout tee. I used an aluminum downpipe. I drilled a set screw into the threads so it can't unscrew.

I had one break of at the threads, but they 2nd one is doing well so far. I found it when you bend the aluminum pipe to have a narrower profile, that it's difficult to avoid having pressure on the threads as the aluminum pipe will tend to go every so slightly oval at the top.

My down pipe is just long enough to be effective, but if we hit some big rollers while surfing and it rocks the boat I can occasionally hear the downpipe come out of the water. But overall it's been a great success.

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