I can't say it any better than the other commenters about gathering items and then deciding how to store them. I will say that I used to keep a duffel bag in my trunk but found it was a pain to get at the things I wanted. I'd have to take 10 things out to reach the one item I was looking for.

I now use an organizer like this one and it works much better for me.

That said, here's what I keep in it: Jumper cables, FAK, small air compressor, two canteens of water, heavy duty space blanket, change of clothes, zip ties, para-cord, syphon, basic tools, maps of my area, fire extinguisher, silicone tape, life straw, folding saw, road flares.