Available as an assembled Module and as a DIY project.
This Module is currently available.
Single 6-to-1 mixer or a dual 3-to-1 mixer
A pair of 3-to-1 fixed- gain mixers. The first output is normalled to the second mixer, so, if no cable is plugged into the first output, the module acts as a single 6-to1 fixed- gain mixer. Circles are ins and squares are outs.
These merchants probably sell this module. Huh?