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When I was in high school I tried this, using 3/4 inch pine and sheet aluminum. The top was a solid wooden circle, and the bottom a smaller donut shaped piece of wood. The aluminum was screwed to it and rivited at the seam. Calking sealed up everything nice and tight. Very stuffy, no matter what I tried with corks. I eventually drilled a few holes in the top to help a little. After fussing with it for a few months, the mute I had ordered finally came in and I threw this contraption out.

It's great that you brought up this point because I wanted to tell everyone of a new idea that I have for a cheap mute, particularly for a sitdown tuba.

Go to any sporting goods store and buy a football. I kid you not. Get a football and put it inside the bell of a 3/4 tuba. For a larger one, you'll need to get a volleyball or basketball and put it inside the bell. But don't put it in too much since you'll have a hard time dislodging it.

I have previously suggested that for the sousaphone, which is my instrument of specialty, that you put in the volleyball or basketball inside the bell so as to practice your instrument quietly.

Good luck on your mute though.

Hank74