I use saddle skirting, which is usually about ten-to-fourteen ounces in thickness. I've found the thicker, stiffer leather tends to keep its shape better after being wet-formed. I wet-form my plates with tap water, temperature doesn't matter, and it hardens the leather well enough to take a beating. After I wet-form and dry my pieces, I apply a finish to the front and back, so that the formed pieces won't soften or lose their shape if your armor happens to get wet (sweat, weather, blood, gore).
ELAD