How to ensure 45 degree movement of object

How would I ensure that my object continues diagonal movement after a collision with another object with the bouncy material.

The object starts of perfectly bouncing at 90 degree angle of rotations but if it hits an obscure angle or a weird collision it changes its degrees of rotation.

It is only bouncing off rectangular objects with box colliders and has starting added force of:

         Vector2 startingDirection = new Vector2(45,45);
         rigidbody.AddForce(startingDirection *this.startingspeed);