An anti-theft device is provided comprising a frame comprising at least three interconnected sides. A plurality of interconnected frame members define the at least three interconnected sides of the device. A longitudinal extent or a vertical extent of one of the interconnected frame members is slidably movable within a bore of a longitudinal extent or a vertical extent of another of the interconnected frame members to a desired position, thereby allowing the device to be adjusted in one of a horizontal or vertical direction. The plurality of interconnected frame members define at least a rectangular-shaped region on each of the at least three interconnected sides.