```groovy
public interface ExistingThing {
    Property<String> getSomeProperty()
}

abstract class DefaultExistingThing implements ExistingThing {
    DefaultExistingThing() {
        getSomeProperty().convention("some-value")
    }
}

// in plugin
existingThing = objects.newInstance(DefaultExistingThing.class)
existingThing.convention("some-value")
```