Hi Rollo,

reading your request I went up to the attic and searched for my turban, but alas - I didn't find it. So I have to try to remember the measures.
The other links sent to you concern female turbans which definitely don't fit a delegate from Harun's court. The turban is an oblong sheet of cloth, about 150 cm long and about 20 cm wide, with fringes at both ends. There are lots of ways to wind it around your head; I only know the way they do it in North Africa, where I learned it during my studies.
Put a short end over your head, so that it covers your neck. Before your forehead give it a twist and wind it around your head, giving it every time a twist at forehead and neck (so it will hold better). When finished, tuck the end into a fold; the fringes may hang out.
Since you are impersonating a delegate to Charlemagne, let put me one thing straight: The embassy bringing an elephant as gift to our unforgotten Emperor came from Harun ar-Rashid (Aaron the Righteous), thence Commander of the Faithful at Badhdad. So the people were not Moors, but pure Arabs. I doubt whether they wore turbans at this time; it seems to be a Turkish custom, and the Turks entered the Arab service some 300 years later. It would suit you better to wear a kafiye with an aqall.
The kafiye is a square sheet of cloth, some times with fringes, too. It's folded once in triangular shape, put over the head and fixed with the aqall, a double ring made from camel hairs. One end hangs down over your neck, the other two over your shoulders. There are different ways to tuck them to the aqall. Especially in dusty weather tey are wound around to cover mouth and nose. I think there are enough pictures of Arabs in American periodicals whre you can see them.
Note: Arafat's kafiye has a coloured pattern; the peasants over there are wearing them in red, green or black and white. The nobleman wore plain white (Hedjaz custom), since the nobles of the court came from the Hedjaz and were, as they called themselves, Arabs of pure stock.
A book about (Moroccan) materials with costumes is:
Beiträge zur Kenntnis der materiellen Kultur Nordwest-Marokkos : Wohnrat, Hausrat, Kostüm / Ernst Rackow. - Wiesbaden : Harrassowitz, 1958
Unfortunately it was stolen in the Oriental Library I was head of, so I can't make a copy of the pictures.
It is also in the Library of congress, LC Call No.: GT377.M6R3
I hope I could be of some help.

Wilfried