I reworked this original fan holder case to fit inside my K40. It should fit any of the "standard" chinese models, but YMMV. FWIW, I got mine in 2021, not sure if they vary at all over time.
This fits a standard 80mm fan and wedges in between the metal braces inside the front cover. It is kind of hard to describe, hopefully you can see from the pictures. This lets it slide it back and forth to cover any part of the area cutting, or just leave it in the middle if it is a big job. I was going to hang it but couldn't get anything that would not leave the lid open - this design happened by accident and works nicely for me, so sharing with you all.
FYI - I used a MT3608 to power it but could definitely wire to the power supply instead.
K40-80mmFanAssistMount-v4.stl | 18.1KB |