I used stainless, scrap I rounded up at work. I put the holes in with a drill press and a 1/4" bit. 24 in the inlet section, went straight thru with the bit , gives me total of 48 holes. Did the same on outlet section, total of 96 holes. The baffle between the two sections was a piece from inside the holesaw when I drilled the end plates. Used a chop saw to cut halfway thru the internal pipe, inserted the disc in the slot and tig'ed it up. It sounded fairly quiet until I mounted the blade. Sounds pretty throaty bouncing back at me. I contemplated filling the 1/4" hole form the pilot bit in the baffle. I ended up leaving it, but believe that's the major source of the noise. I can still reach it with a stainless stick welding rod if it really gets on my nerves.