MIXIPLE

Original schematic: LINK 

This module is a unity gain MIXer and also a buffered multIPLE, so I call it MIXIPLE. Sound weird though.

SCHEMATIC

This schematic shows only a single MIXIPLE. If you are going to make a dual MIXIPLE like what I did, just simply duplicate it. Otherwise, you can use TL071 for U1 instead of TL072. 

STIPBOARD LAYOUT

I omitted diodes such as 1N4001 for reverse voltage protection. If you are worried about it, please add diodes.

FACEPLATE LAYOUT