Having checked the filler plug could actually be removed from the access hole I cut in the gearbox housing I made up this cover plate.

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I lined the inside with a couple of strips of self amalgamating tape to provide a waterproof seal when tightened down.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone