DH made ours using sleepers. Kept sides the full length then cut others in half for the top & bottom so rectangles in shape. Ours are 2 sleepers high which is great for our backs.
He bolted them for strength and braced the shortened ends. Have had the first lot a couple years with no problems.
We did exactly the same thing, got untreated sleepers direct from a sawmill. Plus the sleepers are thick so we can sit on the edge of them. We built 4 no dig gardens and they are going great guns!