Custom motor mixer for VTOL

Hello,

Please point me in the right direction. I have built a pretty normal VTOL from a plane with four (non-tilting) lifter motors. As I tilted the motors in certain directions (3d printed special motor holders) to help with yaw authority, I needed to change the motor directions of all four motors compared to what Ardupilot expects. Instead of spinning CW my left front motor spins CCW and so on. Apart from this, everything is normal.

So far I have learned that I have to setup a custom mixer table, or at least modify the existing one. But how do I do this?

Thank you in advance,
Wolfgang

H frame type is a standard quad with rotaions reversed

This is it! Man, so easy. Thank you!