Designed this fan adapter for my S4U. It is sized for a standard 80mm PC fan which fit very well over the printer's 75mm exhaust port. The fan adapter mounts outside the printer, bolting through the mounting holes into the optional radiused inlet. The fan and duct adapter funnel mount through to the adapter plate. Use 4x M3x22mm machine screws with nuts to mount the adapter plate and inlet (~11mm without inlet). 4x M3x22mm machine screws are also used to mount the fan assuming the pc fan is 10mm thick. Adjust as necessary for other fans. The intended design involves putting captive M3 nuts on both sides of the adapter plate but as seen in the photos this orientation can be reversed, with nuts on the outside. 6-32 screws can be made to work if you enlarge the holes slightly.

The duct adapter funnel has an outlet ID of 52mm, originally designed for using 2" OD central vacuum pipe as ducting.

The fan adapter has a small cutout in the bottom to route any unrelated wiring outside the printer (not needed for this project but I also have a heated vat).