2D colliders for a rotating 3D cube

Basically, when I rotate a 3D object in a 3D space I want a 2D character to have an accurate collider to be able to run on an object. I put a 2D collider on a 3D object by creating two empty children objects with a 2D colliders on them with one rotated at 90 degrees but this can only get me so far. The issue obviously appear anywhere between 0 and 90. How can I do this correctly? At the pic you can see the cube rotated at 45 degrees (his 3d collider shown with green lines), and two white-sprited collider gameobjects as a result.