3d collider mesh to 2d collider mesh

Hi all, I am experimenting with unity's 2d features. I am trying to use a 3d model with 2d backgrounds, and having problems with the collision detection. I understand that the 3d colliders do not work with the 2d colliders - they are using different engines. Is this correct ? I have found a method by attaching 2d colliders to the 3d game object, but this is not ideal as when the 3d object rotates, the shape is different. Question is ; is there any way of converting a 3d collider mesh to a 2d collider mesh ? Sort of projecting the 3d mesh onto a single plane.. then you have a 2d mesh ? Is there a built in way of converting it from 3d to 2d ? Is there an extension to do this ? Thanks for any help..