Everything in a car boot ends up in the same place eventually: the far corner, under the parcel shelf, on its side. Here are the six items that get there fastest, and what actually stops them.
A boot is a smooth plastic tray with nothing to grip. Every corner you take moves the contents, and the things that move first are the ones with a small footprint and a high centre of gravity. That is not a character flaw of your shopping. It is physics, and you can work with it.
1. The reusable shopping bags
They collapse, so they slide. They slide, so they end up under everything else, which is why you keep buying new ones at the till.
The fix is not a bigger bag. It is a fixed home: one crate or one bin that lives in the boot and holds the folded bags upright. Upright matters. Bags stacked flat migrate; bags stood on their short edge stay where you put them.
2. Bottles
A single bottle lying down will roll the full length of the boot and back on every roundabout. It is the loudest object in the car and the one you can never reach.
Bottles want a divider that is taller than half the bottle. Anything shorter and the bottle simply tips over the wall. If you carry bottles regularly, a compartment with a non-slip base is worth more than a wide open crate, because friction underneath does most of the work.
3. The first aid kit and the warning triangle
Both are legally required in several European countries, both are used roughly never, and both end up buried under the things you use weekly. That is the wrong way round: the two items you might need in a hurry are the two hardest to reach.
Keep them at the side or in a side pocket, not in the main pile. If your boot has a recess under the floor, that is the right place for the triangle and the wrong place for the first aid kit, because you will not lift a loaded boot floor with cold hands at the roadside.
4. Cables, chargers and the tyre inflator
Cable ends catch on everything. Two cables in the same space become one knot within a week, and the knot is always tighter than it needs to be.
Roll each cable and hold the roll with its own strap or a hook and loop tie. Then stand the rolls in a compartment rather than laying them flat. A flat cable roll unrolls the first time it slides; a standing one cannot.
5. Wet or dirty things
Boots, a folded camping chair, a dog towel, a bag of garden waste. The problem is never the item, it is that once something wet has been in the boot, everything in the boot smells like it.
The rule that works: wet and dirty things get a container that closes, or at least a container with sides high enough that nothing else touches them. A separate crate you can hose out is worth more than a fabric organizer here, because fabric holds what you were trying to contain.
6. The things you bought and forgot
A parcel you meant to return. A jacket. A single shoe. These accumulate because a boot has no visible edge between storage and rubbish, so nothing ever looks out of place.
The cheapest fix is a hard boundary. Give the boot one container and one rule: if it is not in the container, it leaves the car this week. It sounds mechanical. It is also the only method that survives a busy month.
What actually holds things still
Three properties do the work, and price is not one of them.
Friction underneath. A rubberised or non-slip base beats a heavier box. Weight helps only until the first hard corner.
Height of the divider. A wall shorter than half the object it is holding is decoration. Measure the tallest thing you carry and buy for that.
Something to attach to. Most boots have lashing eyes or hooks at the corners, and most people never use them. One strap through two eyes stops more movement than any organizer on its own.
Measure before you buy
Boot floors are rarely flat and almost never rectangular. Measure the width at the narrowest point, which is usually between the wheel arches, and measure the height to the underside of the parcel shelf if you want to keep using it. Then check whether your spare wheel or your boot floor still lifts with the organizer in place. That last one catches people out more often than the width does.