The PSU of my MK3 pinter used to become very hot, very quickly when printing.
While building the IKEA Lack enclosure from https://www.prusaprinters.org/cheap-simple-3d-printer-enclosure I decided to do a slightly different and activly cooling PSU holder.
This is the first outcome.
Please be aware that there is a left and a right hand mounting (prusa original mount is right hand). The air outlet is different for the left hand one.
For me, the cooling with a low rpm fan worked better when left hand mounted.
The Holder is attached to the Lack with 2 6x30mm screws.
To attach a fan you can use M3x30
I currently have https://www.bequiet.com/en/casefans/266 attached, because of the "Rubberized fan frame for less vibration" :-)