Searching for a better fan duct, i found this this (thx to xander1)

https://www.thingiverse.com/thing:4124587

The initial tests weren't bad, but the airflow didn't fully reach the rear of the model.

So I copied the design and gave it a bit more length and air baffles to get more air behind the model. Thicker walls give the part a little more strength against the heat.

In addition, I added another holding element for the original fan bracket (Prusa R4 / R5 / R6 Part).

Since the PINDA holder tends to lower itself at high printing temperatures, I added a reinforced notch to support the probe.

My parts are printed from Prusament PC Carbon Fiber to withstand higher temperatures. It prints great !

I think the ASA is sufficient for normal prints with PETG and PLA.

It is best to print 3 or 4 perimeters, much infill ~50% and supports at the air outlets as shown in the picture (no supports for PC Carbon Fiber needed ! )

The fan mount could be print with PETG.

You need:
1x M3 Squarenut (use the existing one)
1x M3x8mm
1x M3 Nut (use the existing one)

Testing in progress, Parts could be changed ;-) > i've printed many times with one part > no Problem at all.

Check Out my other Prusa MK3s Things and my Prusaprinter.org Prints
https://www.prusaprinters.org/de/social/1247-kalleknall/prints

Update 04.03.2021
added a MK3S+ Version

Update 18.08.2021
renamed the MK3S+ file and updated the file to the latest Iteration (only small changes for PC Carbon Fiber printing).