I remodelled the original design which had jagged polygons making the surface uneven. I also included the FreeCAD file so you can modify it freely.

Update 24.09.2020: Addedv004. I improved (I think) the internal part where there were columns crossing all the way over the fan, thinking that it is blocking the airflow I removed them. It is much cleaner now. Also, I started to get better with FreeCAD so the new file has less nodes and it is much more cleaner than before. Later, I'll do some CFD Simulation to see if this really improves the airflow.