Sorry rock331 I was mistaken, I thought you had been talking about the little tin plate that fits over the hinge to hide the heavy steel strap. If you got real lucky, someone may know of a partially rusted cab that has that area in tact and repairable.
When I was building mine I found an old truck that had been abandoned in the woods at the back of an old farm. Like you I could never find an aftermarket stamped out repair pcs. Sorry I couldn't help.