Leak-proof, and will fit both regular and perforated flasks. Great for Centrifugal casting and vacuum lost wax casting processes; made from natural rubber materials, and holds 5/16" - 3/8" tree tightly into place. Please contact us with any questions. We always combine and reduce shipping costs on multiple orders. Please visit our store for other great refining and jewelry tooling items such as; Melting Furnaces, Safety Gear, Crucibles, Molds, Stamps, Tongs, Borax and Much Much More! Thanks for Looking!