By the most convincing accounts I've read, the original great kilt was a lengthy piece of unpleated and unsewn handspun and handwoven woven wool that was soaked in melted goose grease and then worn under the most extreme Scottish conditions, meaning amongst heather, brambles, and such drenched with dew, plus in general rain, fog, cold, mud, mixed with liberal quantities of manure from cattle and sheep and, of course, blood. People slept on the ground, out of doors and overnight in them and, if in groups, spread one out as an improvised shelter- the Great Kilt was a rough and tumble garment to end all such garments. If I wanted to wear one under authentic conditions, I would certainly choose wool since almost anything else would let you down under wet conditions, perhaps fatally, and I would also be shopping for, or rendering, goose grease... but under ren fair conditions as I imagine them (I've never been to one), surely any fabric resembling wool would do? I would of course test the fire retardancy before draping myself and then walking around amongst people who were smoking the herbs of Araby....