Tom, the latches on the box can't be locked and we don't stock anything like that. However, it would be very easy to add your own hasp, either riveted or welded on the box.
Answered on 12/27/2012 by Clyde from NRS
I had some tabs riveted on to mine for setting the box in the frame by a metal shop and have had no problems. I see no reason why you couldn't rivet on a hasp as well. Just tell whoever you have do it that it needs to stay watertight.