To mirror one needs to make an extra sub group or component of the object, with its sizes set or controlled, then you make the copy size negative (lenx, leny, lenz).
However there is a need to redraw if another attribute is changed.
mirror handles.skp (185.1 KB)
will look in to trying to stabilize the example, maybe an extra level of subs…?
But much easier to create mirrored objects and hide them as suited