I use leather, about like that used in a baseball glove. One of the secrets of keeping it positioned, other than what has already be mentioned, is to hand select flints. I carry a small hand make gage, and try to pick the ones that fit, width, length, and have nice consistent shapes. I know if you're ordering them, then it wouldn't be possible, but if you do have the opportunity, to hand select, it will make changing them out, and staying seated a lot easier.