This thing replaces the two belt clamps on the Prusa v2 X-carriage with a unibody clamp with a tab for mounting a 40mm fan and duct.
Included in the files are the mount and duct STL files, and the OpenSCAD source file that generated them. If your printer needs a different size for these parts, edit fan-mount.scad and regenerate the STLs.
Using your favorite slicer, plate the STLs and generate the code for your printer.
Send that G-Code to your printer and print it like A Boss. Use SAE 4-40 or M3 screws to bolt everything together. See the pics.