The problem is down to the case itself. It is not designed for great airflow. As all the pics on the internet are of the front, so I don't know how many fans you have at the back.
Either buy some decent fans, as OvenMaster said, and leave the side (partly) off.
Or, buy a new case which is designed for good airflow. So, grills at both back and front (a side grill isn't really necessary).
The second option will provide better temperatures, but will cost more.