I mocked this up when the heat from my Antminers got to be too much for my basement. They bolt nicely into the standard 120mm fans and are the perfect size for 4" standard flexible ducts used for household dryers.
I designed the adapter in Imperial measurements, so make sure to double check your settings before printing a very small adapter.
EDIT: I have redesigned it slightly to fit the original metric units. It should be a perfect fit now for a traditional 120mm fan and a 4" duct.