If you are planning to do the other container interior, I found a couple things that made my ceiling interior siding easier.
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Third Hand Adjustable supports
you can see them in use in this reel from doing the interior of my horse trailer with FRP panels.
and for supporting the end of the panels, I screwed scrap pieces of plywood into the furring strips loose enough that i could rotate them 90 degrees to catch the end
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I think you made something really nice out of something a lot of people go cheap on and end up unhappy with. I like your idea of a roof ala "Pole barn" between the two for additional shade and storage. I'd put the doors on the inside of the "courtyard" for a nice common area.
Following after getting caught up. Keep up the good work!