A simple pencil clip holder based on one I received about 20 years ago for my euphonium.

I made multiple sizes for euphonium, trombone and tuba.

The 3D model can be found here if you want to create your own : https://cad.onshape.com/documents/0562a30502107c09c25a6284/w/0c55cf8e7b60d20e879c6904/e/df9f87371f615afcc1ec7de0