The shape of the corner would most easily be done with a full size die in a stamping press that blanks and forms the whole door panel. A small scale example is sheet metal cover plates for electrical outlets.
Doing a small number of parts would be more practical with a different design. I suppose low volume could be done by notching the corners, then using a press brake to curl the edges, then welding the resulting gaps at the corners and hand finishing the weld beads.
Looks like an expensive part any way you do it.
Larry