You do NOT want a long skinny box that screams, "Firearm - STEAL ME!!!"
Also, long skinny boxes are more easily damaged. They WILL stack something on it and it will bend in the middle.
You want a shorter rectangular box. Split the upper from the lower, and wrap each half in lots of bubble wrap. Reinforce both ends of the upper with 3-4 layers of cardboard to prevent the ends from breaking through and being damaged. Nothing is worse than receiving a firearm withe muzzle sticking out of the box and damaged.
2" wide fiberglass strapping tape is your friend. Use lots of it.