A device for locking the door or flap of a typical pick-up truck shell closed from the interior of the shell includes a substantially rigid body which is selectively secured to moving parts of a latch mechanism. The device is designed to operate with an existing shell door or flap latching mechanism of the type having a rotatable shaft coupled to a pivotable plate. According to such existing latching mechanisms, the pivotable plate is pivoted (or rotated) between first and second positions. In the first position, the plate partially overlaps a portion of the rear wall of the shell (or the truck bed or tailgate) to inhibit the shell door or flap from swinging open. In the second position, the plate does not overlap a portion of the shell (truck bed or tailgate), so as to allow the shell door or flap to swing open. In order to lock the shell door or flap, the substantially rigid body is secured to the pivotal plate of the latch mechanism from the interior of the shell. The rigid body has a body portion which overlaps a portion of the shell when the pivotal plate of the latch mechanism is in the second position. As a result, the shell door or flap is inhibited from swinging open, even when the pivotal plate is in the second position.