I hate that design, do not make the sides sink in llike that. (My current sunbeam core contact cooler does that and it is a horrible design. due to the design of the fans most of the airflow is directed around the edges of the fan, and when the side of the heatsink is not a uniform thickness, you end up with inefficacies.
the area before the indentation remains cool and gets a lot of airflow, then the area after the indent (where the fins get wider again) they remain how and get very little airflow from the fan)
Using tape to make a box to even out the width of the cooler and box in the gaps for force all of the air to go through the length of fins dropped my CPU temperature by about 10-12C over what the heatsink can already do on it's default config.