This currently popular ‘Gun Metal’ (deep shiny grey colour) plastic base, metal cap tack button measures 17mm in diameter. Strong and easy to apply by pushing a ‘tack’ into the barrel cavity with a small press. Careful use of hammer is also possible for sampling.
Generally, a solid aluminium tack (rivet-like pointy companion) with a barbed tip will give a fantastic closure and very long and reliable service.
Can’t really go wrong. Idiot-proof.