How do you make a circular one-sided collider?

I am making a space ship game where every ship has an elliptical or circular shield surrounding it. I need the ships to shoot bullets that can pass through one side of the shield, but will collide when attempting to go back through. Using a Circle Collider 2D is not preferred because of lack of support for elliptical shapes.

I have attempted to use multiple edge colliders and a single multi-pointed edge colliders in correspondence with a Platform Effector 2D, yet I would get very inconsistent behavior on what side would allow objects in.

Is there in any way possible in Unity 5.0.1 to create a collider (or composition of colliders) that would exhibit this behavior? Am I simply misunderstanding the mechanics of the Platform Effector 2D?