Wall Backer Plate
A 110 x 70 x 8 mm wall anchor plate from the 'NEMA17 Hanging Wall Polargraph Drawbot' kit (print two). It fastens to the wall with four #8 wood screws in counterbored 4.8 mm holes on a 90 x 50 mm pattern. The spool standoff cage's four 12 mm posts drop into 12.4 mm square recesses on a 54 mm square and are held by M4x10 cap screws driven from the wall side. Those screw heads sit in 8 mm counterbores, below the flat wall face.
robotics kit polargraph nema17