A composite core assembly includes a composite core structure having internal material interfaces and an attachment rail coupled to the composite core structure. The attachment rail includes a first planar surface and a second planar surface adjoined with the first planar surface. The first planar surface is arranged parallel to an internal material interface plane of the composite core structure, and the first planar portion is at least partially integrated into the composite core structure in an internal material interface plane. At least a portion of the first planar surface extends beyond a perimeter surface of the composite core structure, and the second planar surface is configured to attach to the surrounding support member. The core material does not have net edge facets or flat edges positioned next to surrounding structure, and/or may not have a structure arranged parallel to the adjacent support structure to attach to the support structure.