You can't have enough mixers in your modular! This is a 5-channel input mixer with two identical outputs. Note that the outputs are identical and there are no buffering between them. I used 1M potentiometers for the input level controls which might not be optimal, but I have a lot of nice and small 1M potentiometers lying around so I wanted to use these. The mixer will give two times amplification.
Unfortunately there are no building instructions for this module, but at least you'll find the schematic, some photos, and some comments.
Two (almost) finished modules.
There is one trim potentiometer to adjust for DC offset.
One mixer mounted in the modular
That's it! Have fun and mix your sources!