My very first Prusa still works on in the shop and I happened to rest my hand on the power supply the other day. It was really hot! So I designed this.
This part has two holes to mount the 80mm fan and then just slips over the top of the silver PSU. (It does not fit on the black PSU as typically mounted.) It was designed to print easily and not use unnecessary material.
I printed it in ABS at 0.2 mm layer height, 2 perimeters, and 20% grid infill. The holes are designed to have M4 cap head screws self tap and hold snug.
I'm using a 12V, 80mm fan I had laying around, and just ran some wire from my ATX breakout board to power it (independent of the printer).
You probably want to cover the vents in the PSU that aim towards the build plate. [see edit] I used a strip of black electrical tape.
EDIT: The electrical tape I used was really old and the adhesive failed after a few prints. Now I'm using aluminum duct tape (which I had lying around) instead.
The author hasn't provided the model origin yet.