It's an fan adapter which takes a 120mm fan and directs the airflow 90 degrees.
It's kinda a special model as I used it do direct air from the front cabin of my VW T6 back to the storage area, over the dividing wall. But the design is rather general, so I'm putting this in here in case any other could use it for whatever.. :)
The author marked this model as their own original creation.