The strap tension is the model. A compression sack is not a bag with straps on it — it is a bag whose entire shape is produced by the straps, and if the fabric does not pull in where they cross it, the prop looks like a cylinder with belts painted around it. Here the compression is baked into the silhouette: the sack is drawn in at four points, the fabric bunches between them, and the ends bulge where the contents have nowhere left to go. That reads correctly from any angle, including from directly above, which is where most faked versions fall apart. Strapping and buckles are separate geometry. A small, cheap, unglamorous prop, and the one that makes a camp scene look packed rather than staged — kit that has been compressed is kit somebody intended to carry.