Hi Dan,
I'm actually working on the same thing myself. You can find my linen tiara/kybarisia on this forum if you search for it, and on the Bronze Age Center I'm
posting progress on my scale armour. I'm actually considering starting a thread about Achaemenid tunics to see what other early clothing is known from areas where people wore tunics and trousers. I'm also at the stage of looking at art and wondering how to turn it into a pattern.
The book "Forgotten Empire" from a British Museum exhibition has lots of useful pictures of art including fragments of frescos from Susa. S.I. Rudenko's book on Scythian finds from the Altai Mountains has the pattern of a Scythian linen shirt and mentions a felt tiara/kyrbasia which he doesn't sketch or include in the photographs
Erik Schmidt's books on the Persepolis excavations have some interesting things too.
Here are my thoughts so far on Persian clothing.
The ordinary male outfit was the 'Median dress' of cap with neckflap and chin flaps, knee-length sleeved tunic, sash, trousers, and soft leather shoes. This is depicted in many sources such as the Persepolis reliefs, gold plaques in the Oxus treasure, and the Alexander Mosaic. Unfortunately, art doesn't tell us about many details, such as how the trousers were held up or what was worn underneath. We can tell that the outer tunic is narrow at the wrists, fairly close about the chest, and wide below the waist to allow walking. There is no sign of a vertical slit at the neck, although the neck opening does look fairly tight and people are rarely seen from the front in art from the empire.
The most promising text is from Strabo, where the geographer gives an antiquarian description of the Persians:
Strabo, Geography, 3.15.19 (from Lascus Curtius, tr. H.L. Jones): They serve in the army and hold commands from twenty to fifty years of age, both as foot-soldiers and as horsemen; and they do not approach a market-place, for they neither sell nor buy. They arm themselves with a rhomboidal wicker-shield; and besides quivers they have swords and knives; and on their heads they wear a tower-like hat; and their breastplates are made of scales of iron.
The garb of the commanders consists of three-ply trousers, and of a double tunic, with sleeves, that reaches to the knees, the under garment being white and the upper vari-coloured. In summer they wear a purple or vari-coloured cloak, in winter a vari-coloured one only; and their turbans are similar to those of the Magi; and they wear a deep double shoe. Most of the people wear a double tunic that reaches to the middle of the shin, and a piece of linen cloth round the head; and each man has a bow and sling. Persians dine in an extravagant manner, serving whole animals in great numbers and of various kinds; and their couches, as also their drinking-cups and everything else, are so brilliantly ornamented that they gleam with gold and silver.
That sounds to me like a wool or linen undertunic with sleeves and a wool overtunic with sleeves. There is some disagreement between Greek art, which shows Persian tunics as baggy, and art from the empire, which shows a smoother, tighter look. I'm inclined to follow art from the empire, although the truth was probably in the middle. Someday I should look at the finds from the Persepolis excavations for possible buttons and buckles.
Sean