Order a cheap plastic case from Amazon. They ship it to you in a perfect size box. Put the gun in the case, zip tie case shut, back in the box, tape it up and ship it.

"Life is not a journey to the grave with the intention of arriving safely in a well preserved body, but rather to skid in broadside, used up, worn out, bottle of Jim Beam in one hand and a .45 in the other, loudly proclaiming WOW-- What a Ride!"