You should add trapdoor support, so lets say you make a cashier area, no one but you can get in.

If you put the trapdoor next to a wall, you can also put the sign on the wall above the trap door (I believe).

Deadbolt already supports trapdoors - just right-click on a trapdoor while holding a sign. It will look strange (as the sign is taller than the trapdoor itself), but it will work as you requested.