I'm going to address moisture first because it will affect EVERYTHING in your 72 hour kit, or bug out bag. Moisture is bad news to just about everything you want to store in your kit. In your first aid kit, it can be detrimental to medications or pain killers that are stowed. With food, it can advance the spoilage to just a few days rather than a year. With electronics, moisture can render even the coolest GPS receiver into a paperweight. With maps, documents, money, and other paper, moisture can make the ink run, the paper tear, and make writing impossible. With clothing, it dampens them and advances mold growth, which renders them useless and unsanitary. My point is that moisture is the enemy of all stored emergency items.
You must take great care to rid your kit of any moisture before you stow it away and make sure that moisture stays away. How do you do this?
First off, I'm a big fan of reusable desiccant bags. They draw moisture away from your gear and can be recharged in the oven or microwave, depending on what you get. I prefer the oven versions because of who knows just what happens in the microwave! The one I linked to is easily identified as needing to be recharged because it gets harder as it absorbs moisture. Just to be safe, you might as well get one not only for in your 72 hour kit box, but put one in the trunk as well as one under your seat in your vehicle. Remember, there is a good possibility that you will be able to drive straight home in most emergencies or disasters. Your vehicle is your first line of defense. As long as it has a tank of gas, some basic emergency tools, and isn't molding out from under you, it will serve you well.
You will also want to store smaller desiccant packets in with your medicines. Most pain killers and prescription drugs are designed to disintegrate when they come in contact with moisture. Unless you want to open a small packet of paste when you go for your favorite pain killer, I suggest a small watertight baggy to store them in (separate from the rest of your first aid kit) and a small desiccant packet inside with it. Packets, such as these are a good choice because they change color to let you know they are unusable.
This brings me to another part of moisture abatement and the laws of physics, if you will. I know, BORING! Man, physics is so dumb! Yeah, it may be dumb and boring, but it is the law and all things must abide by it.
When a container is warm, and then opened in a cold environment, moisture will collect. When the container is closed again, moisture droplets will form. With nowhere else to go, moisture will absorb into anything that will absorb it. Don't believe me? Place a regular dry house hold sponge into a small bowl of water about the same size as the sponge. Come back a few hours later and you will find that the sponge as absorbed almost, if not all, of the water. That's what your gear does! But that is also what a desiccant pack does and it will attract moisture more than your gear will, which will keep it dry and usable. Now, I'm not saying to drop a desiccant pack into a bowl of water. The bowl analogy is a macro-sized example of what happens on a really REALLY small scale. If you've ever left your cell-phone in a car and came back to see moisture in behind your screen, then you know what I'm talking about.
Let me switch gears and talk about a box. What kind of box you use for your 72 hour kit is up to you, but a few basic rules apply:
1. It must secure tightly. It doesn't necessarily have to be airtight or watertight, but being very moisture resistant helps a lot.
2. It should be strong enough to get banged around in the trunk or the back cargo area of your vehicle. You should be able to stand on it without collapsing it. If you cannot, then you need a stronger box.
3. It should be large enough to accommodate all of your gear when packed in, but not so large that it becomes a burden and you find yourself leaving it home. Remember a survival kit left at home is no survival kit at all; it's just a collection of things.
4. It should be low profile enough as to not attract unwanted attention. Don't go labeling it SURVIVAL STUFF or 72 HOUR KIT, or something like that. These kits are a significant investment, and it would be a shame to see a year's worth of kit buildup go away in an instant.
5. Bolt it down if possible. Lock it always.
Okay, so I digressed from sole moisture abatement, but I figured there was no sense talking about a box unless I just got that information out there. Now you know.
Your box should be free of holes. Don't drill drain holes in the bottom of your box. That only serves to attract any moisture that may gather in the carpet UNDER the box. Yes, it happens. I have a tool box in my Ramcharger that was actually stuck to the carpet because moisture collected under it and froze in the cold air. It happens. If you are concerned about moisture building up in the box, put a 1 lb desiccant bag in there and sleep better at night. By the way, a 1 lb desiccant bag is good for an entire trunk, so sealing it in a small box is plenty!
We must now move on to smaller packaging. You want your food to stay fresh and not spoil. You want your electronics to stay dry and protected. You want your batteries to stay in working shape. You want your clothing to remain dry and mildew free. You want your boots to be dry. You sure want a lot. The desiccant bag in the box is going to do a lot to keep your gear nice and dry, but for some items, you need a little extra protection against moisture. Additionally, smaller items will still need to remain moisture free when they transition from the box to the backpack. If you think a car is a moisture prone hellhole for food and gear, wait until you hump it 15 miles in a driving rain.
I like using zip-loc bags in various sizes. I like the freezer bags because they are stronger than your average sandwich baggies. Freezer bags come in all sizes, from smaller to uber huge! You may even vacuum seal some items that you know won't go bad. Batteries last for years in storage. Vacuum seal them with a desiccant pack, date it, and don't worry for about 3-5 years.
If you store food, use FDA approved desiccant packs. No, this doesn't mean to store desiccant packs that can be eaten. There is no such thing. Just use a pack that won't contaminate the food you are storing. If you are trying to save money and store homemade dehydrated fruits, veggies, and jerky, then an FDA approved desiccant pack is worth its weight in gold. Just remember that proper rotation is absolutely necessary with food items, especially homemade goods.
Whenever packing loose food products, vacuum seal it. It removes the air from the package (most of it anyway). The lack of air/oxygen inhibits bacteria growth no matter how much moisture is in it. Rotation on a six month to annual basis, depending on the item is critical. Do not store raw meat or fresh fruits and veggies. They will spoil quickly.
Another option is freeze-dried foods, as they are oftentimes sealed with nitrogen or some other inert gas that renders bacteria growth a non-issue. Most freeze-dried foods last a lot longer than dehydrated foods do. As long as you don't break the seal, you need not worry about moisture entering these foods.
The bottom line is this: if it sealed in a package, drop a desiccant pack in it. Layered defense is your best defense in moisture abatement. Start small. Drop a desiccant sachet in each of your first aid baggies, and the kit. Put desiccant packets into small bags that store other gear or food. Put a large 1 lb desiccant bag in the box to keep all other gear dry. Put a desiccant bag in the trunk or cargo area outside the box to add an additional layer of defense. If you want to really go crazy, put a couple 1 lb desiccant bags in the passenger compartment (hidden of course) to keep moisture at bay there too. Another added benefit is you might find that your car windows don't fog up as much, which is especially nice in older vehicles.