Look for a rubber plug to slide over it and hose clamp on. It should be made of the same type of rubber as your radiator hoses. I don't know where to look, maybe napa or ace hardware
![I Dunno... :dunno:](https://board.marlincrawler.com/Smileys/marlin/dunno.gif)
We run caps like these but bigger on our houseboats, but they're a factory omc part so I don't know where to find something like it.