I recently printed Neilberticus' two-piece dual extruder fan duct. It printed out really well but I was concerned about the strength of the tab where it screws to the carriage. So I made this little retaining washer. It is the same size and shape as the mounting tab on the extruder fan duct and it holds the screw firmly enough that it keeps the screw from falling out and it reinforces the duct mount.
If you install one of the 3D printed fan ducts, give this a shot, to help strengthen that screw mount.