McMaster-Carr has some pvc sleeve in various sizes, including 3/8". Couldn't find much info about the appearance of the material itself, but at 45 cents per foot, ten feet would be less than five bucks. Lots of other types of sleeving too, including metallic braid, high temp ceramic braid, basic nylon braid, etc, might make some nicely detailed coverings for other portions of the harness. Wall thickness on the 3/8" sleeve is listed as .025".