Here you go.

Here’s a simple stereo summing algorithm.  Will mix 4 stereo inputs into a single stereo output.  All knobs are in dB.  0dB is full volume, no attenuation.

Choose whichever 8 inputs you want and whichever 2 outputs.

https://godlike.com.au/index.php?id=420

I haven’t tested it yet, but it should work.  I’ll upload the compiled algorithm next time I log into my studio.

Enjoy.