A special mini amp channel switcher/FX loop pedal (When you actually only use 1 of the 2 functions and the other is always on or off) Details in comments. : r/diypedals
Amp channel switch. LED question
KIT guitar pedal A/B switch box (2 IN, 1 OUT) - DIY PEDAL GEAR PARTS
How to Add LED Lights to a Guitar Amplifier Foot Switch Pedal DIY Marshall DSL 40c Mods LED Lights - YouTube
DIY: OT Pickup Machine footswitch (now with final pics!) - #84 by Markel - Octatrack - Elektronauts