Toon Outline Shader + ortographic camera?

Hi,

I make a little 2D platformer with Unity. I have a cube as the stage for the player and want to add a Basic Toon Outline shader to it to draw the black outlines. But it doesn't work when I change the camera to ortographic. In the editor view the outline is only drawn in the backside of the object, so I can't see them with the ortographic camera.

This doesn't work for me:

 v2f vert(appdata v) {
     v2f o;
     o.pos = v.vertex;
     o.pos.xyz += v.normal.xyz *_Outline;
     o.pos = mul(UNITY_MATRIX_MVP, o.pos);
     o.color = _OutlineColor;
     return o;
 }

Is there are workaround or a other shader to fix this? I only want a outline at the four visible edges of the cube.

Tuni