How do I mirror across anything other than X, Y, or Z axis? I'm trying to mirror an object on the X-axis, Y-axis, and X+Y-axis. Unfortunately, there's no check mark for that last one. How do I mirror along it?
Use two mirror modifiers, both one along the X+Y axis.
Once you have the Y and X mirrored once, then you can add an empty and use it as a mirror object to mirror it along the x+y for your second modifier.


Simply position the empty to where you want to objects to be mirrored. The empty will essentially replace the object's origin for the mirror modifier.