Im assuming the safe wont be in a climate controlled room. Im assuming the safe will be inside, at least semi protected from the elements. Im picturing a locked up hunting cabin?
I lived in Valdez, Cooper Landing, Seward, Anchorage and Soldotna for a few years. I do miss Alaska.
Not being a metallurgist, or plastic...urgist, I wonder if, since they wont be used, the plastic under even intense cold, would be fine as it wouldnt be flexing and the change from cold to hot wouldnt be quick enough to cause damage.
Moisture and oxygen would be my primary concerns.
A light coating (can see fingerprints in, but does not pool or collect) rust inhibitting grease over both the exterior and interior surfaces, coupled with an oxygen and moisture absorber, physical protection such as bags or cases could serve you well. Storing with the barrel pointed down can help avoid excess oil and grease pooling in your action, gumming it up even more, if in a standup gunsafe or room.
Im of the opinion that grease serves well for storage and oil serves well for lubrication.
Cosmoline in a plastic bag in a box crate worked well for some folk. A protective plastic case with custom cut foam padding, pressure release valves, wheels, etc etc would probably be nice.