This is an adapter I made to be able to use an old 120 mm fan as solder fume extractor. The pipe is 100 mm external diameter, the internal diameter is (IIRC) 94 mm.
If you want to use it with a connector to a "real" 100 mm ventilation duct, you need to change the pipe internal diameter, which should be relatively easily done since I included the FreeCAD files.
The holes in the adapter fit 5 mm nuts/bolts tightly, or 4 mm comfortably.
Clas Olson PLA