I was just thinking about this last night. In one of the end of the world books, it wasn't One Second After, I think it was some other series called the last light or something like that. Anyways, the guy used PVC pipe to bury thousands of rounds of ammunition unbeknownst to his wife. With the talks of new firearm bans, I figured I should scrounge up the materials to do this in the worst case scenario that I need to bury some of my firearms/ammo.
To the OP: Perhaps a Pelican type case? While expensive, they are very resistant to extreme weather changes and would probably do the job very well.