The No Entry Unauthorized Personnel 8116 sign will make it abundantly clear that people that do not have the necessary permission will not be permitted to enter a certain area. This will ensure that an area is only accessed by the correct personnel.
As an added bonus, the sign would even glow in the dark if there were no lights, to carry on advising the relevant people. This is because the sign is Jalite AAA photoluminescent.
Fixing requires anything as simple as sticky pads, 'No-nails' type adhesive or double-sided tape.