Edit 13.03.2023: Added slim version that is one part print.

Stock fan is not really powerfull so improving it can only be a good thing right?

I was quite surprised how much the stock part cooling fan on ender 3 pro is blowing air away from the inlet and so loosing quite some efficiency - try yourself and put a finger next to the fan and see what i'm talking about :). Well, of course it also depends on the fan duct you are using and its resistance so results will not be the same for everybody.

The part is designed to be glued on the fan body - I know it is something not everybody would like to do but few tiny drops of superglue will not harm anything and could be easily removed in the future.

For non slim version at first, glue the little ring on the bottom of the velocity stack, it serves as spacer so the fins of the fan will not rub against it. Then glue the velocity stack on the fan - be carefull to place it correctly in line with the fan inlet.