A DIN rail mounting for a Sonoff basic to fit into a standard breaker box. Because the raised face of the Sonoff is slightly too wide, it has to live below the face of the breaker box when closed.
The Sonoff is fixed to the mounting with M3 bolts - the holes are undersized on purpose, so that you cut a thread in the plastic as you fasten it.
FreeCAD file included in case you want to modify the holes or the height or anything else.

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.3mm