If you don't have the original box, I recommend two options:
1)Container store has a some unique shaped boxes. There is one that is a big bigger than needed for a rifle or shotgun, but it will work fine.
2)If you want more protection, buy a hard plastic case on amazon. It will come in a box. Ship the rifle inside the plastic case INSIDE the box. This is how I have shipped rifles. With no writing on the outside, no one can tell it is a firearm. Very good protection.
If you do find a rifle or shotgun box from a manufacturer, wrap it in brown crate paper to ship. Nothing like a big green 'Remington' on the side of the box to advertise what you are shipping...