The first Prusa MK3's were shipped with Noctua fans with hideous colors. Hence I made a cover for the Noctua fan as found here: https://www.thingiverse.com/thing:2730049
When Prusa released the new MK3S parts, some modifications to the cover had to be made, to fit the new Prusa parts.
The principle remained the same: The cover has some notches that will "snap" into the rear bolt heads and the front is kept in place by 2 M3 bolts going through the cover - just look at the pictures it should be self explanatory.
The top M3 bolt is 18 mm long, the bottom M3 bolt is 25 mm long - these can be retrieved from the supplied spare bolts.