Low Tech room indicator?

Looking for an idea to keep people from knocking on the Bishops door during interviews. What have some of you seen? Any suggestions? I think we are going to make an LDS themed door hanger, but interested if any of you have other ideas.

One of our buildings has a little light by each office that the bishop can switch on. It says something like "Interviewing, Do Not Disturb". I don't know if they came with the building or if the FM Group installed them later, but when the light is on, hardly anyone ever knocks.

Another method is to station the executive secretary near the door to intercept people who are trying to barge in.

Our building used to have two Bishop offices where you could only enter through the ward clerk offices. They did a costly remodel (which took several weeks) to move the doors so that people could come and go without having to be seen by people in the clerk offices. But we usually don't have problems. People know that if the Bishop's door is closed and the clerk's door is open, they can easily ask someone in the clerk's office whether the Bishop is available at that time. Of course, if the clerk's door is open and there's nobody in the clerk's office...

We have the door with a small window in it. So we can see if bishop (Branch President indeed) is in and if it's alone (ready to be "bother" from us)!