If you are building something to fit in a vehicle, space and weight are fairly minor considerations.
If it were just for the car, weight is not an issue. Size may to some degree; not weight.
If we had to ditch the car weight then becomes an issue.
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.
That is how my EDC bag came to be.
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.
I know. The point of the proposed kit is to have something I can learn.
Jeanette Isabelle