Fred, I was wondering about that, too. Perhaps a nail that protrudes from the bottom of the patch box lid that prevents the lid from sliding all the way out (while keeping the edge of the lid still within the guide rails.)
Another idea would be a strip of slightly-curved spring steel mounted to the forward end of the bottom side of the lid, that curls away from the bottom surface, and catches on the back of the box cavity before the lid is completely open. To remove the lid completely, just slide a thin blade under the lid to keep the spring from catching.