Drill a 9,5 mm hole in the left side of the lid. It’s plastic so it’s cheese-easy.
Put a good quality mono switched audio jack socket (neutrik or switchcraft are OK) and screw the nut tight.
Solder the jack’s ground wire to the left tip of the volume slide-pot.
Desolder the middle wire of the slide-pot and connect it with a wire that goes to the switch tip of the jack plug.
Protect the connection inbetween wires with a piece of heatshrink tube.
Wire the signal tip of the jack with the middle tip of the slide-pot.
Use a shielded piece of cable for best resulting sound.