I'm glad you asked this.
I'd like to ask your help, and give you an opportunity to help the forum, as well.
A member, smarter and better looking than I, brought these to my attention..
Fammocan.com - Home
I don't have one yet, but the concept is you slide this into a standard military ammo can, which can be had for $4 to $7 bucks.
Instant waterproof, indestructible case.
Further, Cheaper than Dirt offers an Ammo Can locking system.
If you go this route, I'd like you to post a review. It helps this place, as this is a product not alot of people are aware of.
Give it some thought.