I have two large kits.  One in each car.  The one in my wifes truck is designed to stay there in or very close to the car.  It is housed in a plastic toolbox-type container.  The other kit (my largest) is in my truck and is designed to be quite portable and serve as both a kit for my car as well a BOB to support the entire family.  It is housed in a standard size day pack.  
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Learn to improvise everything.