I have modified and shortened Hook3D PSU switch's STL and located it on the left hand side of the ENDER 3-Max.

Also, I added 2 brackets to hold the PSU fan down to avoid all the debris from fan's suck in force.

All you need is:

1. 4 X M4 to hold the PSU tie on the brackets
2. 2 X M3 or M4 T Slots + Screws to hold the PSU brackets to the 4040 Aluminium Profiles
3. 3 X M3 or M4 T Slots + Screws to hold the PSU Switch Housing to the 4040 Aluminium Profile