I want to mirror object using it's face as a mirroring plane. For the example, I want to use the selected face on the cube as a mirror plane.
I created a custom transform orientation using that face, but is not the one which gets used by mirror modifier. I also positioned a 3D cursor at that face and then transformed origin to it, but then rotation is wrong (from local axis). I also see I can set mirror object, but my face is not an object. I added an empty, but it's orientation is not aligned to the face either.
I am out of ideas now. I think best way is to set local axis to that custom transform. I will be happy to do it anyway, because local axis are completely off and the object is hard to work with.
Result using the method suggested by MrZak and lemon: