trombone mouthpiece holder 'mack' substitute

thingiverse

This is a useful tool for practicing only on your mouthpiece whilst having the instrument in your hands. There is one version for standard large and medium bore. It does not block the mouthpiece when not used (see picture). This concept is known as 'Gerd Dowids Mack' for trombone Should be usable by other deep brass instruments, too.