On August 4, 2004, about 0805 eastern daylight time, a Cessna 208B, N1037N, operated by Superior Aviation, Inc. and piloted by a commercial pilot, was substantially damaged when the cargo door opened during cruise flight and a box departed the aircraft impacting the horizontal stabilizer. The pilot diverted to Roben-Hood Airport (RQB), Big Rapids, Michigan, and landed safely. The flight was operating under 14 CFR Part 135 on an instrument flight rules flight plan. Visual meteorological conditions prevailed. The pilot reported no injuries. The flight departed Gerald R. Ford International Airport (GRR), Grand Rapids, Michigan, approximately 0750, with an intended destination of Cherry Capital Airport (TVC), Traverse City, Michigan. The pilot reported that after loading the cargo, he and the loaders had to "muscle the cargo door closed due to a large volume of freight." He noted that taxi and takeoff were normal, but that a few minutes after leveling at cruising altitude the door ajar light illuminated. The pilot stated that "shortly after this illumination, there was a loud thud and [he] figured that a box had departed the aircraft and struck the tail." The pilot reported that aircraft control was normal after the accident. He stated that he employed the door ajar checklist and diverted to RBQ. He noted that he landed uneventfully. When he exited the aircraft, the pilot saw the damage to the left horizontal stabilizer. A Federal Aviation Administration inspector examined the cargo door installation on the accident aircraft. He reported the door structure and locking mechanism were intact and appeared undamaged. The door closed and latched properly. The inspector noted that the door had a window installed. He stated that with this design the door had enough flexibility to allow the forward frame of the door to be seated in the door jamb and latched, while the aft frame was outside the jamb and unlatched. He reported the micro switch for the DOOR WARNING annunciator light was located on the forward jamb. Therefore, it was possible in that configuration for the aft cargo door latch not to be engaged without the annunciator light being illuminated.