Nonetheless, I was able to get a portion of the retaining wall up. I was originally planning on wood planks, but I saw that corrugated metal siding was sometimes used in the trenches so I though it might be fitting here as well.
I might be taking liberty of some artistic licensing as I've not seen any pictures of this exact set up before. I have seen pics of big gun emplacements by hills and even in the woods so I don't think it is out of the question.
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One thing I have not been able to figure out is what color the corrugated siding was. Was it painted, galvanized? Would it rust? most pictures I've seen show it being bright compared to the surrounding dirt or wood.