You can buy plastic bags specifically made for vacuum storage. They are popular for seasonal clothing storage. I would put the coins in capsules, the capsules into pill bottles, the bottles into a heavy plastic ammo box, then vacuum seal it. Not sure of your motives, but coin condition makes a huge difference in price, so a little extra work can more than pay for itself. You are also dealing with a dilemma. Keep it secret, or trust someone? All the coin caches found are due to secrecy, and not being able to access before passing. But if you share, well, silver is extremely visible to metal detectors! Heck, it could be found even if no one knows but you.
Army Surplus store often carry waterproof, plastic ammo cases with a rubber o-ring you might consider along with some silica gel packs.