Protective polycarbonate enclosure with open back box and exterior key lock for flush mount applications helps protect a wide range of larger size devices, components and instruments, such as keypads, annunciators and automated external defibrillators (AEDs) which require protection from vandalism, accidental damage, as well as the elements, such as dirt and grime. For indoor or outdoor applications, the enclosure allows the protected unit be clearly seen and quickly identified. The enclosure is formed from tough polycarbonate material, which is backed by a three year guarantee against breakage and damage in normal use.