Abolt's advise is right on, but as you purchased the sep funnel from auction i assume you prefer to not deal with lab/glass companies.
So if seeking a ghetto replacement, look in big rubber stores (they will have rubber stoppers), wine spirit and brewery suppliers (they have em for their drums and tubs etc.., or even as corks for different bottles), or carve one yourself if your bored. Cover it with teflon tape so it doesn't absorbe solution. You can even use your gloved palm or similar if you're in a real dilly of a pickle