I wondered, which fan-upgrade would be worth it, so I decided to take a more powerful 5015 fan - the EBM PAPST RLF35-8/14N 24V.
I remixed the original BEAR Fan duct, so credits to Bear Lab https://github.com/gregsaun/bear_extruder_and_x_axis/tree/master/source and thanks for the great shroud!
The fan duct is not optimized using software, but printing several versions and looking, how to center the air directly under the nozzle. I am pretty impressed by the results I got so far - I also reduced the fan speed for non-bridge lines. I included the Step file, so feel free to remix! Ahoo!
BOM:
The author remixed this model.
Changed nearly everything of the fan shroud to fit the RLF35