Paper-based elongate structures suitable for use as lumber substitutes, articles of manufacture made thereof, kits of materials including such structures, and machines and methods for making the same. Elongate structures according to the invention include a hollow outer portion formed of corrugated fiberboard sheet material and an accordion-folded inner support formed of corrugated fiberboard sheet material. The hollow outer portion has four outer surfaces that define a rectangle when viewed in cross-section transverse to a longitudinal axis of the elongate structure. The accordion- folded inner support is enclosed within the hollow outer portion. The inner support includes a plurality of first spaced-apart peaks that contact a first inner surface of the hollow outer portion and a plurality of second spaced-apart peaks that contact an opposing second inner surface of the hollow outer portion. The accordion-folded inner support structure improves the beam strength, crush resistance and lateral strength of the elongate structure.