If you can't find gun box I have just cut open a box and folded it over the rifle and duct tape it up good. News paper ball around the muzzle and butt. Bubble wrap it if its in nice shape gun. milsurps or beater guns don't need bubble wrap.
Canada post wants you to lock it inside the box even though you can just put the key in box. One worker told me they x-ray boxes and open up the gun boxes if its not trigger locked. My experience says that is a lie.
They also say signature needed but in my experience this is also a lie. I have had a rifle in a box on left on my front step a couple times. I once shipped a shotgun and the worker guessed right that it was a gun and told me to trigger lock it. I opened it up, trigger locked it and shipped it with no signature.