I am the queen of hauling my whole life around in the car/truck – well, hauling around my day, anyway. For decades. Drive distances daily. Multiple changes of clothing and footwear. Saddle or two. Backpack of helmet & riding stuff. Food. Drink. Etc.
Plain old Walmart plastic crate totes are very conveniently sized. You can line up several, or just a few.
Tall narrow tubs with lids also fit well. I have one (without the lid) on the floor between the passenger seat and the dash. Mini-crates in the bottom and a car-carrier on top of those make it multi-tiered for items I use throughout the day. Easy to live life in the driver’s seat. Of my car, I mean.
IMO keep in mind the height of the container you need, to keep things from spilling out if you suddenly must brake heavily. Rarely, the setup in the front or back seat will have a sudden slide forward. They may even tip over toward the floor, against the back seat. But everything in the crate stays in the crate, so it’s still just as accessible. Also none of it slides over into my driver’s area.
Containers of liquids - thermos & so forth - also need taller sides on a container, as they tend to fall heavily and with little provocation.
Aside from the rigid containers that hold it together in the back seat or back trunk/storage area, I believe in backpacks (Walmart/Target ones are fine) to gather everything by purpose. Everything for riding, everything for another activity. And a separate scheme for winter and rain wear.
I am rarely caught out without what I need. It has been too miserable when that happens, so I’ve become good at being sure.
Car is out for repair, or I would grab a photo of my crates loaded with horse stuff that goes back and forth between two barns.
I have a major post out in the COTH world somewhere with my entire work-barn-totallife-in-the-car scheme. One or two or three years ago? Not sure it’s findable now.
One more note – I tend to have multiple big projects and life directions going on simoultaneously. So, lots of different stuff for each, because they don’t overlap. If you keep life more simple than I do, carrying necessities with you will become less complicated, too.