A tamper-resistant threaded fastener including a circular head having a slot formed in the head transverse to the longitudinal axis, the slot having an enlarged region intermediate an apex and a base of the slot. A cap member includes a head having a drive feature formed therein, the head extending to a first shank portion, extending to an enlarged second shank portion, extending to a foot. The cap member is insertable in the slot only from a direction transverse to the longitudinal axis. During rotational installation of the fastener and cap member in a threaded counterbored opening for securing a barrier for protecting an asset, the foot shears inside the slot at a predetermined rotational drive force, the cap member being rotatable in the opening and protectively covering the fastener head.