Knock says it "does not affect ropes, vines, and the like" ...
... so just TIE the darned thing shut. Bar the door, and tie the bar in place with normal, ordinary rope.
Also note, Knock doesn't bypass traps associated with a door; if you can open the door with teh key and NOT disarm the trap, then, knock will unlock the door, you open it -- and suck trap. Put the trap disarming mechanism in a hidden wall niche near the door, and that's another deterrent.