A friend asked me to build him a simple volume control for his his in-ear monitors. All you need to have to build one is some stereo jacks, enclosure and a potentiometer. Potentiometer value should be 5-10 times the headphone impedance and have a reasonable power handling (1W+). You shouldn’t run a hot signal to it anyway since the pot can literally burn!
Here’s how it turned out. I used a tiny enclosure so you can use a belt clip to attach it (for example) to a guitar strap.