I added a skull on a plain guitar knob. In case you really want to go metal ;-) I printed mine in Azeure Film's silk silver.

The knob should fit easily but thight enough to twist the button. If it's too loose: there's a hole in the back where you can add a tiny screw. Or you can add some superglue if you're sure you'll never swap it. Or wrap some tape around the tiny stick on your guitar to make it a bit wider so it's tighter. Anyway... It fits for me without these solutions, so I assume it'd fit for your guitar aswell. Mine's a Yamaha RGX btw.

• Size: +- 32.5mm x 22.4mm• Layer height: 0,15 mm• Infill: 15%• Support: see picture• Print time: 1 hour