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
USED Bright Onion Pedals Looper with Amp Channel Selector - Loop Switcher Pedal - Matt's Music Center
How to Add LED Lights to a Guitar Amplifier Foot Switch Pedal DIY Marshall DSL 40c Mods LED Lights - YouTube
Amazon.com: Vivlex ABY Switch Line Selector Selection Channel Pedal Mini AB Box Switcher Electric Guitar Amplifier Footswitch True Bypass : Musical Instruments