To build a survival kit, you can put in stuff you think you might need based on what you have available or what your buddy Ted has or what you saw on that reality TV show. This results in ‘a’ survival kit, but generally not the ‘best’ kit. There is a difference between ‘building’ a kit and ‘designing’ a kit.
To design a survival kit, first you need to answer several of questions.
1) How big can the kit be? If it is a kit which you will have with you ‘at all times’, then it will obviously need to be very small and light. Conversely, if it is a kit to be carried in a heavy duty vehicle only when heading off-road, then size and weight is much less of a concern.
2) What do you want to be able to survive? The most common answer, ‘anything and everything’, is not helpful in selecting survival kits, since no matter what you think of and are actually able to prepare for, I guarantee that you won’t think of something, and you won’t be able to practically prepare for everything you do think of. One process is to think of the most dangerous likely occurrence and include solutions to that, then the next most dangerous or likely, and so on until you hit some limit (size, cost, too low a probability).
3) What are the cost constraints? A survival kit is ‘insurance’, not a ‘purchase’. What can you afford to spend for something which may never be used, and which may need to periodically have some parts replaced due to expiration? Note that removing items from a survival kit to use in everyday circumstances should mostly be avoided, since until the item is returned or replaced, the kit no longer meets its design criteria.
4) What legal or practical restrictions are there, if any? If you often have to go through security checks, always carrying a big knife or even a bunch of small metal objects with you may cause problems. A uniform or other specialized clothing or corporate policy or local law may limit what you can carry at times.
5) What packaging will you use? Generally, you want any survival kit to be in a package (or set of packages) which allow the kit to be ‘grabbed’ without searching for any part, which can be easy transported by whatever methods the kit is designed for AND which protects the contents so that they will be ready for use when needed.
Often, the best methodology will be a ‘modular’ approach. This starts with a ‘micro’ kit and/or ‘pocket’ kit you have with you whenever you are outside the home (‘EDC’ or Every Day Carry). In your car could be an appropriate add-on kit for when you are driving. And if you ever go into the ‘wild’, a portable add-on kit (either part of your car kit or part of your hiking/camping equipment) on your belt or in your pack would be wise. There are more extensive concepts, such as ‘bug-out bags’ (sometimes called a BOB) and ‘caches’ as part of the ‘prepper’ disciplines, but these are beyond the scope of this investigation, since their design criteria is independent living rather than short term survival and attracting rescue.
So, the goal of these kits will be to survive unexpected, localized circumstances until rescued or the circumstances pass. This could range from being stuck in an elevator during a power outage or being a victim of a natural disaster, to an accident or breakdown on the road, to being lost or hit with unexpected weather conditions while on the trail. As you can see, the gear which would be applicable to each situation might vary, so kits designed for those circumstances not only have different packaging limitations, but a different focus as well. It is possible to have a variety of kits for various possibilities, but the costs of duplicate equipment may be prohibitive, and even if not, the odds of having the right kit when a particular event occurs is disturbingly low. Again, modularity can help out. Have a kit with all the basics for ‘any’ circumstance, and then you can attempt to have the appropriate add-on module or modules for the most likely occurrences based on your plans at any point in time.
Let us say you complete a design. You still do not have a survival kit, just plans for one. The plans need to be turned into reality, and that, sadly, is not guaranteed to be successful. In fact, there is a very good chance it will not be successful. Something you are counting on may no longer be available, or has skyrocketed in price. Whatever packaging you choose is almost always too big or too small. In short, arriving at the desired kit is an ‘iterative’ (several pass) process. Design, assemble, evaluate failures then redesign, reassemble, etc. Redesign may involve removing something or adding something, or replacing something large or costly or of limited use, with smaller, cheaper and/or more versatile items, or repackaging things.
Eventually you will end up with the best compromises which seem to satisfy your original goals. But even then it is a good idea to step back and evaluate what you have come up with. Sure, you have everything you think you need (suitable for the size of kit), but is everything really necessary, is anything critical missing which just ‘seemed too big’, is the quality and packaging of everything adequate? And lastly, do you have the maximum practical versatility for each item in the kit? Remember that the versatility of your survival kit contents is a major factor in how effective it will be.