On the original DC-10 cargo doors it was possible to move the locking lever into the locked position without having the (electrically operated) latches fully engaged.
After the accident the door latching mechanism was redesigned. It is now impossible to push the locking lever down if the latches are not in their end position, due to a cam as part of them, which has to move out of the way for the locking bars. The position of the locking bars can (and has to be checked) through small windows inserted in the cargo door. The same door is in use on the MD
-11. There have been no more problems since them.
Since the pressure differetial pulled down the floor over the aft cargo hold and pulled on the control cables, causing the flight crew to loose control over their plane, there have been several blow out panels incorporated in the cargo hold ceiling and sidewall panels, which will permit a controlled equalisation of pressure if there should be a decompressation either in the cabin or cargo holds.
BTW, the 747 had a similar problem with a cargo door opening in flight, killing several people (United Airlines). Only the fact that the control cables of a 747 run above the cabin (due to it´s cockpit in the upper deck), prevented a similar loss as the Turkish Airlines accident. The lock and latch mechanism of the 747 was redesigned as well, with similar inspection windows and a change of the material of the locking cams from aluminium to steel.