I think the sack up would still need to be in a waterproof/resistant bag. If it got wet, I think it would be worthless. A good coating of car wax (be sure to take the gun out of the stock and coat the bottom of the barrel and action but not moving parts before leaving and then check your sights) and care to wipe the exposed surfaces down with a rust preventative/water displacer every night on the hunt would be my suggestion.
I actually didn't htink about the Case.. I was jsut gonigto Fly them over in a Airline aproved HARD metal case and then at the Airport in Saskatoon carry them on the Placne. I think thats how it worked ? MAybe just have a Scabbard of sort. At night, Although not overy parinoid, I think the rifle will lay nicely next to me in the tent and not outside.
At home I have big safe with a De-humidifier in it. It keeps the moisture out of it well.