You can see the behind the bridge circular port the two times where the Reliant flies away from the camera. (After Khan retreats the 1st time, and then after the phaser exchange in the nebula.) Otherwise, you're right as there are no more circular ports.
The model has a rectangular port on the portside rim of the saucer.
You could put the extra ports in the side of the wedge afthull and hide them behind some retracting plate doors