If I really kept kitted out for an incident like this I use up all my grocery room or most of the back seats. I was in my last car, but because I usually didn't take passengers.
If you optimize for space it's not too bad. Even a thin blanket of the right material is a huge improvement over nothing. That and nest things inside of each other where possible and/or use vacuum seal bags. I've typically been able to jam everything in the least convenient corner of the trunk.
