Summary
Plans and more details here : http://therandomlab.blogspot.com/2015/05/simple-passive-monostereo-to-stereo.html
This one does not match the schematic exactly. It has two stereo channels and one mono channel into one stereo out. Used 10K potentiometer, 1K resistors, and 1uF Capacitors.
Mini Passive Mixer 3D Print Model
