In flight, one strut is the bottom, and the bridle is tied between the top and bottom of this strut. The dihedrals of the sails help stability.
Large box kites are constructed as cellular kites. Rather than one box, there are many, each with its own set of sails.
Most of the altitude records for kite flying are held by large box kites, with Dacron sails, flown with Spectra cable. Before Dacron, Spectra-cord and Kevlar were available, high performace box kites used oiled silk, linen or hemp sails, and were flown with steel cable. Silk, linen and hemp were used because they could be spun finer than cotton and stretched relatively little when wet. Steel had the highest available strength for its weight.
See also: kite flying