Somehow the previous design must have been a fairly efficient one as Red Bull have been using the round brake duct (left of image) since at least the beginning of 2008. As of that design, Red Bull have always consistently built their ducts from Windform XT, a rapid prototyping material often used to create the models tested in windtunnels.
The latest design is from the same material but changes the shape of the air inlet. The outer part, closest to the wheel is now (right of image) in line with the tyre wall. It still points down so that it exactly matches the direction of airflow behind the front wing's elements.