The door on our 1970 mounts just as you are showing. There is a piece of aluminum extending out from under the trailer body which the hinge is riveted to. In the closed position, the aluminum piano hinge is doubled over and in the open, it is up 90 degrees.
I would think you could do the same by attaching an aluminum strip across the back down through the frame rails. I would wedge it as best I could under the body, then rivet to it.
Below is a picture before I started the restoration of the original mount arrangement.
Craig and Carol
1997 34' Excella 1000
1970 27' Overlander, International
2009 Ford F150 5.4L
ProPride hitch with 1400# bars
Southeastern Camping Unit (12)