A quick suggestion about adding a little fitting tolerance to allow all kind of basic printers to achieve some results. I printed both duct and motor-mount-rev2.on a 350 euros CR-10 printer
1- I did add some water + soap but It was still super hard to screw one-in-another. Maybe a printer default on my side (I used 0.3mm accuracy to improve waterproofness). So what I did was to rub both duct inside and motor-mount-rev2 outside so both could fit. So maybe a good idea would be to “eat” both duct and motor-mount-rev2 “teeth” by a extra radius of 0.1mm.
2- When I tried to fit the screw perfectly to close the last 2mm screwing gap for a perfect fit), the last conic part of the duct popped out. No big deal, it can work that way, it just made an extra part But may I suggest that the very top part of the motor-mount-rev2 to be shorten by say 2 to 3 mm (when looking at Flo drawing, this is the very top part)
Now that said, this very top part is the one laying on the glass when starting to print, meaning that this is the bottom layer, and maybe that bottom layer was too thick on my printer settings.