Sportsman's Guide sells "vaults" suitable for burial in various sizes. They look like pvc, but appear to be better in construction, although pricey. If you are looking to store for an extended period, remove the grips/stock and fill the pvc with new oil to store actions. If the action is submerged in oil it will not rust. Grips and stocks could just go into a watertight PVC setup, no oil.
I would use the PVC and PVC cement to make it waterproof. Of course, this would not be practical for short term, but the PVC with threaded caps and dessicant would work.