Ray rotation with edge of character rather than centre.

I have a shape that I can rotate and move forwards/backwards. I can draw a ray from the centre of the shape which works fine as I move the shape but if I want to draw the ray from the one edge of the shape I get problems when I rotate. Here's the ray coming out of the object on the extreme edge:

Here's if I rotate 90 degrees:

These lines draw the ray:

 float left = shapeCentre.x - (shapeSize.x/2);
 float x = shapePos.x - left;
 ray = new Ray(new Vector3(x,shapePos.y,shapePos.z), transform.forward);
 Debug.DrawRay(ray.origin,ray.direction);

When I rotate object the ray rotates but maintains it's origin not reflecting the changing position of the outer edge of the shape as it rotates.

I only move in the X and Z directions and I can see that I need to recalculate the x and z vectors to include the rotation and direction to establish where the edge of the shape moves as it rotates. The problem is that I can't work out how to do it, can anyone help, or is there an easier way to do this.

Thanks sd