A plurality of members for temporary assembly into a rigid branch structure for supporting a cover to form an enclosure, including a plurality of elongated rigid, hollow posts defined by opposed pairs of flat post walls joined together along their respective marginal edges between spaced-apart terminal ends, the walls having formed therein at least one pair of apertures in spaced-apart, opposed relationship near each post terminal end and a series of elongated ribs extending inward from the walls and along the lengths of the posts between the apertures, a coupling for axially joining one post to another in rigid, axial, end-to-end alignment including a butt plate for transverse positioning between the terminal ends of the joined posts, at least two stems extending outward from the butt plate for insertion a distance into each hollow post end, each stem including two pairs of closely-spaced webs arranged in spaced-apart configuration and defined by opposed marginal edges spanning between the opposed walls of the post and having formed between the closely-spaced webs a slot for sliding receipt therein of a pair of opposed ribs when the stems are inserted into the hollow post ends, a cross-web transversely intersecting the spaced-apart pairs of webs and an end plate arranged parallel to the butt plate and intersecting the terminal edges of the webs and the cross-web to rigidify the stem and, a pair of arms of terminal length, extending downward along the stems, exterior thereof, and arranged to slidingly engage the interior surface of the post walls, the arms having formed thereon outwardly disposed tabs for securely fitting into the apertures formed in the opposed post walls after the arms are biased inward and the stems are fully inserted in the terminal ends of the posts.