Most belts and girdles in this period seem to have been made of rope or textile and knotted not buckled. When Xenophon Cyropaedia 6.2.32 speaks of the need for straps (ἱμάντες) he is talking about load-bearing equipment not what holds in your tunic. Curtius Rufus, a Roman from a buckle-wearing culture, read a description of the Persians knotting their belts and declared that this was effeminate (Historia Alexandri Magni 3.3.18). Roman men in his day sexualized the sash like 20th century Americans sexualized the bra (Catullus, Carmina, 2; cp Standard Babylonian Epic of Gilgamesh, vii.156).
The sculptures at Persepolis have great depictions of knotted Persian belts!
Adult male burial IB 134 at Nippur: “adult male medium contracted on right side, oriented northeast; cloth shroud, cloth sash at waist, traces of leather perhaps indicating shoes, breeches, shirt and cap … a burial (Pl. 160 A) in which traces of leather shoes, pants, and head covering suggested the type of dress worn by the Medians or certain far-eastern subjects of the Achaemenians”: McCown et al., Nippur I, pp. 128, 146, pl. 160a; Potts, “Disposal of the Dead” p. 269