I like USGI ammo cans. Sturdy, water tight if you maintain the seals, and cheap ($5 to $15 depending on what you get and condition) and they are pretty easy to rig a shoulder strap on. They are a bit too bulky for a PSK but behind the seat of a vehicle they work. And you can sit on them. If you have two one makes a seat and the other a compact workbench/work surface.