If you are building something to fit in a vehicle, space and weight are fairly minor considerations.

In any event, assembling a survival kit is an ongoing process, building on user experience and incorporating new items as they appear. I continually tweaked my SAR bag as time went on and got it to a pretty reasonable state. Then it was stolen, and I got to start all over again. But at least insurance covered most of the cost.

You can't just buy stuff, put it in a bag, and forget it. ideally you will have familiarity with items in your kit and know how they perform. An emergency is not the time to be reading the manual for the first time.