I just use a piece of 1/4" siphon hose. I sanitize it, let it dry then put about 3 inches into the neck and let the rest dangle. Then I stuff the funnel in and pour away. When dry it also grips the funnel and keeps it from wandering around the neck of the carboy while you pour.
They also sell funnels with raised edges molded into the insertion side of the funnel. This creates the air gap you are looking for.