Nice but why did you have some plastic casing over it? wouldn't it be better without any casing?
It's just a duct, to direct the air onto the cards instead of it going everywhere.Airflow from that fan without the plastic duct would look like this:
http://www.youtube.com/watch?v=8m8fC809TK0&feature=youtu.be&t=47s
Compared to with the duct it would look more like this (but not exactly):
http://www.youtube.com/watch?v=8m8fC809TK0&feature=youtu.be&t=1m47s
^yup
it just helps force the air to cross the heatsinks where its needed most