My son is going to be renting a room but currently his room has an antique door knob with no lock. Is there a way to secure the door when he leaves the room? It's an antique I think they call it mortise, with a skeleton key type key hole. He's been told he can't alter anything so adding a dead bolt isn't an option. He needs to be able to have something that won't need to be installed but that he can lock when he leaves and use a key to get back inside.
Buying a new knob is an option, but look at the photo? Is it possible to find a new keyed knob that would fit that without making any modifications?
Your last idea, using the triangle...can't wrap my head around that at all. Can you provide a sketch maybe?
That's what I've had in mind. Before contacting you I found this item but its from the UK. Hoped you could figure out something better. The child system is similar. http://www.youtube.com/watch?v=W7Ix5xvcTn8
Basically, it seems there is no real answer outside of this item. Seems like it wouldn't be that difficult but guess it is. Thanks for trying though.
One other thing just came to me...and I admit still not sure about your drawing, so maybe this is similar to yours. Using a rubber door stop, attaching to a chain that passes under the door then wraps around the knob and locks. You'd need a stick or something to push the stopper out of the way via the space under the door once you unlocked the chain though.