Just picture a rectangular plunger plate with a hole in it that sits over the wafer inside the receptacle. Then when you plug in, the plunger is pushed back. Tiny springs push it back out.

I know just enough to know that something like this is more complex to engineer than you want... It's awfully small, it needs to move freely even when crudded up, it needs to not impede plugging in, etc

Of course this won't protect 100% from ingress, just reduce it.