I'm running the light diameter at about a radius.007 to .05 with the strength between 200 and 1000. The higher the strength the further the beam goes.
My light box is a modified cube with a concave in the face done using a sphere and a Boolean function. I then have placed different shutters in front to control the beam width.
I kept having problems with light leaking through the walls of the box until I applied a reflective shader to the box, as soon as I did that the light only went out the front of the box.
I will get the models posted tonight. I say models because I modified your model and took the light box out, and created separate models of the light box (exploring different beam widths) which when placed at the same coordinates as the lighthouse will be at the same position your light box was. The nice thing about separate that you can rotate the lighthouse to best suit the scene and then point the light box model any direction you would like. The light source would be set at the same x,z coordinates as the lighthouse with a y value of 18.706 above the base elevation. At this point I have 3 light boxes that I have created, all are interchangeable.
Sorry I don't have any images to post yet will do so later today (I hope).