Very cool trick the way that you are mounting that fan.
A simple trick for higher natural convection cooling is to mount the board vertically off the ground to improve laminar airflow. That way there is less resistance to natural air flow (hot air rises) on both sides of the board.
Another improvement to that is to mount a fan directly below the board which is still vertical, blowing cold air up, that way you are cooling both sides of the PCB which is in effect a large copper heat sink. And you are gaining a minute amount of extra cooling aided by natural convection.