This is a picture of the back of my 68 Sovereign
. The frame pieces are available from Vintage Trailer Supply, though I have not needed to buy any yet. Mitered corners, aluminum sheeting inside and out, insulation between. Let me know if you need specific pictures and I can add them later. The design is not great in my opinion because at least on mine, the bottom frame tilts into the trailer channeling water inside. It operates exactly like the access to the fridge and street side storage compartment if you have those to compare. The ones on the side do not tip in like the rear one does.
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