This is adjacent to the mp3 components page. An easy way to add volume control is a pre-built Audio Adjuster. The following look interesting to me:
The KGUSS Mini Active Volume Control Knob would be easy to install in an item. Just drill a round hole the size of the knob itself, and have a panel mounted below the hole to hold it in place.
The Nobsound Mini Active Control Knob also would work with a round hole, but people could try to lift up on it.
You also could drill a round hole for the knob on the Electronics More Volbox Inline Audio Volume Control, and just use a piece of wood to the side to keep it from being rotated.
The Volbox Mini inline volume control might be more of a challenge to mount.
The SPRODIO 3.5mm Audio Volume Controller looks like it is high quality, but also might be more work to mount.