The steel angle is the best/easiest option. Here's a sketch of what I did with a 3mm door and body, external hinges and the inspector was more than happy.
A rectangular frame welded inside the door provides the bracing and with the hinge side of the angle turned outwards there is no way to remove the door even with the hinges removed.
I did place a lock at the top and bottom corners to stop the risk of twisting the door. As mentioned steel surrounding the lock like forestry gates is perfect, Use RHS if you can find some or cut/weld it in the size you need.
Attachment 222731