Hmm... sounds like you might want to try a bunch of overhand knots, using both halfs of the U part of the rope as if it's a single rope (i.e. the U loop end at the top is the end of the "single" piece of rope). Then you end up with something that (horizontally) looks like this: <>X==X==X==X=[scratching board] -- all you have to do is make sure the top <> loop will fit over the door knob.