My solution is plastic boxes with lids, a milk crate, a lift out tray, a hinged shelf, and good organization.
The closed boxes hold things I don’t need often, that I want to stay clean (one is mostly non drug first aid - bandages, syringes, vetrap, etc - and the other is clean cloth - emergency hoodie and socks, clean facecloths, no bows, etc). One sits on the bottom of my trunk and a grooming kit sits on top. The other sits on/slightly in the top of the milk crate (which holds other rarely used items, some of which are in bags or smaller plastic boxes to keep them organized).
The crate and box fit below the tray. The tray holds smaller items needed more often, and my “stuff box” for the really small items (needle and thread, pack of Kleenex, pen, measuring tape, spare Trek velcros, tick tweezers, sharpening stone, knife - you know, stuff
).
Under the hinged shelf are some less needed items (sugar,. epsom salts, hi-vis quarter sheet) and my other horse’s grooming kit sits on the shelf.
There is room at one end of each grooming kit to tuck 2-3 bottles, and the shape of the kits allow my helmet to tuck slightly between and on them.
It also has a bandage lid.
My trunk won’t close if things aren’t where they are supposed to be - incentive to keep it organized!
I’m working on my trailer tack room now that I’ve been using it for a year and have a good idea of what I want to have in there.