Thanks for all the suggestions, but the cones one seemed to me the most appropiate. I had something similar in mind and so I went and made this double cone with sharp edges cut out at the perimeter. I used a film can like in the picture. I pushed this botch-up inside the pipe at the output end, and voila, less noise . Enough to pass the inspection.
Once that is over I can just pull them out with a hook, even if I partially deform them, then bent then straight again and wait to use them next year again.
Stefan Pietzsch
