I the case of "Mound C", here is a photo from one end, with the "two chambers" on the left. Below it I sketch what I take to be straight lines that are visible in the structure.Another closer view of the end of the pile, from higher up, so we can see the hollows more clearly:
In the lower half of the picture, I sketch over what look like straight lines in the structure. For instance the side of the nearer hollow looks straight enough to justify thinking rectangular holes in the pile.Here is a video of the same mound, looking at the side with the collapsed "chambers" from the other end. Note what I call a "retaining wall, fallen out". It forms a straight line along the right side of the mound. That straight line would be on the left in the above photos.
Looking carefully at the pictures, there is other structure on this mound that I do not capture with the abstract mound sketches but is the simplification reasonable?