You said: 2.) Put a sign on the outside of your cubicle warning people that CBI material is being used.
Sorry, that's not a correct answer.
Well, that's certainly a novel way to get everyone's attention. But, it probably won't be highly appreciated by those of us working in the "cubes" and CBI security officers.
Simply placing a warning sign in a non-secure area is not enough to ensure CBI is properly secured.