Abstract: A locking device for protecting the contents of a container from theft includes an extension member having an adjustable length and a lock for inhibiting adjustment of the extension member's length when the lock is engaged. To secure the contents of a container from theft, the locking device is installed between two opposing surfaces, such as between two vertical wall surfaces, with the extension member extending across the container door with the door in its closed position. The locking device is installed by simply inserting each end of the extension member into suitable cavities formed in the opposing surfaces, or in brackets attached to the opposing surfaces, adjusting the length of the extension member as necessary. Thereafter, the lock can be engaged to prevent adjustment of the extension member's length, and thereby prevent removal of the ends of the extension member from the surface openings.