This is a duct for a vertically-mounted 50mm squirrel-cage blower fan. It snaps directly onto the blower. I'm using it for my modified 3DR Simple 3D printer, but it is pretty generic and could be used for just about anything that needs such a part. It works great for cooling PLA.
Some compatible parts:
EDIT: I've improved this model by increasing the size of the nozzle. In the previous version of the duct, the nozzle was too small, seriously restricting airflow. The newer models have the word
Blower- in front of the files. I've left the old ones intact for historical reasons.