Portal Holding lock
If this small metal box is placed on any door or gate made of wood, metal, or stone, it will stick to the door or gate and hold it fast as though it were locked. A knock spell or dispel magic can render the lock useless, causing it to drop to the ground and release the door. A locked portal can be physically battered down, but in suck a case the lock is not damaged and can be used again. The lock activates when placed against a portal and is deactivated by a command word.