I need to set my Headphones Output into mono

I need help with my Quad Cortex Headphones Output. I wanna set it into amono output, I’ve used it for so long in other equipment as an amp route monitoring on stage. My usual signal grid is:

  • 2 outputs to the Soundboard;
    (they could either be from de XLRs outs or the “out 4/r & out 3/l”)
  • 1 output to the amplifier on stage using as a monitor
    (I used to connect thru the headphones output)

That’s why I need to convert this output into a mono, can you guys help me with this?

Thanks, Rafael!