When I added a TrailTech digital instrument, I kept it simple and cheap. Stopped by the local hardware store and purchased a solid rubber, tapered "bung hole" plug. Had the OEM speedo cable and matched a middlin' TLAR size. Tapped it solidly in place and then epoxied a shiny Nickel on the top. Been there for many years now...
Pick a length that doesn't block the vent in the gun-drilled NEG battery cable bolt.