avation, don't waste time with a vinyl insert, use undercoating, line-x, rino, or something instead.  You'll get a better rust protection bond.  BRD HNTER used Hurculiner, ask him what he thinks.  TT uses some other type of undercoating material covered with paint (you could use a color match paint) on their high end custom stuff.  Spray-ons don't squeak, make noise, cause rub spots, etc., and make the bond we all need to prevent rust from the top side.