Available as an assembled Module and as a DIY project.
This Module is currently available.
Simple 2 x 3 input mono Mixer
Dual Simple Mixer with 2 x 3 mono inputs and 2 outputs, DC coupled.
Attenuation pot on each input and one master volume pot.
Vin Max = -10/+10 V
Vout Max = -10/+10 V
http://www.davidhaillant.com/simple-mixer-with-3-mono-inputs/
Bubblesound Instruments Mix 6