Blue Anodized Aluminum Adapter (AN 932), Inch Male NPT Allen Head Plug

For a 1/8 NPT port that needs to be closed off, this is a solid fit. The male NPT threading is sized at -02, which corresponds to 1/8 inch, so it'll thread right into a standard 1/8 NPT female port. The Allen head drive makes it easy to torque down in tight spots where a hex wrench gets in easier than a wrench would.

Its anodized aluminum build is known for resisting corrosion, so it should last, even if things get a bit rough. The Allen head makes it easy to tighten down properly, too.