miauu's Modifiers' Gizmo Align allows you to quickly align the gizmo of the Symmetry, Slice and Mirror modifiers by picking 3 points in the scene https://www.youtube.com/watch?v=RL6gX8D03b8 This a commercial script. You can find more information here.
miauu's Modifiers' Gizmo Aligner version 1.1 is available. + NEW: Added for Symmetry modifier: "Align to Working Pivot's Plane" which allows you to align the symmetry plane to any of the XY, XZ, YZ planes of the working pivot. for Slice modifier: "Align to active grid" which allows you to align the slice plane to the…