I bought this Power Socket Inlet from German Amazon
https://www.amazon.de/gp/product/B07RRY5MYZ
The Rectangular Cutout is designed 27.85mm x 47.85mm
Depth 24.25mm
Its designed so it can be screwed into 2 m3 nuts slid into the frame with 2 m3x10mm bolts
Fasten the Socket Inlet with 2 m3x25mm Bolts into the 2 recessed m3 nuts on the other side
The author hasn't provided the model origin yet.