Put an elbow inside the box pointing down. On the outside of the box, put a T such that the bulkhead connects to the "middle" leg of the T. Orient the T such that one leg points up and the other points down.
The leg that points down is the drain to the sump.
On the leg that points up, put a cap with a hole drilled in it. Put some airline in the hole, and a valve on the end to adjust airflow.
This is basically a "through the wall" style durso, and used by many. Pretty much your best choice for a silent standpipe with a small box and a bulkhead on the back/side of the tank.