i thought of simple Nema 23 cooling assemby made by two 3d printed parts.
the first part is the main body which is mounted with four M3x12 bolts (and nuts) directly in the stepper motor.
The second part "bolt base" are the 'bumpers' attached to between the steppermotor and the M3 bolts
Finally the are four slots in the back where you can mount a 60mm fan
Ready for printing ,no support needed for the print
Step 1: 3dprint
First of all you got to 3d print the .stl files attached to this instructable
Then you need 4 Hex bolts M3x12 and four M3nuts to bolt in the Φ4 holes
Slide the motor around 20-30mm into the base
you can also use a block (of whtaever )30-40mm thickness as a distancer in the back side of the motor to make easier your life.
center it and you re ready for the next step
Now use your extraordinary skills to put the "bumpers" , just slide them nicely in the gap between the bolt and the motor
then start fastening the bolts one by one evenly in all sides
Now you can fasten your 50mm fan in the back side in the for slots and you re to go..