Corresponding blog post: https://burningbecks.wordpress.com/2018/12/25/new-y-gantry-stacking-the-pulleys-again/
xxdanmurphyxx designed a reinforced (left) NEMA 17 motor mount and published it on Thingiverse (as CC-BY).
As the motor shaft position is important for CoreXY mechanics and he didn't mention which other mount to use, I took out the raise here.
Mirroring has to be done in the slicer.
Editable via TinkerCAD => https://www.tinkercad.com/things/jMvWHQnZjyh-counterpart-for-motor-mount-thing3200434
Original Part => https://www.thingiverse.com/thing:3200434