Cabin disorder is often caused less by quantity than by mixed-frequency storage
Cars usually become chaotic not only because there is too much luggage, but because tissues, medicine, snacks and light jackets get mixed into the same retrieval system as main bags, wash kits and evening-use items.
That forces repeated boot access and repeated overflow into seat and foot space.
The shared access layer should handle only same-day, shared high-frequency needs
The cabin layer works best when it stays narrow: tissues, wipes, basic medicine, water, small snacks and perhaps a light extra layer.
Once it starts absorbing hotel-use items or everyone's personal low-frequency gear, usable seating space disappears quickly.
The main luggage layer should own large items, low-frequency items and hotel-arrival tasks
The boot layer is better for main suitcases, wash kits, heavier clothing and reserve supplies that do not need constant access.
That separation prevents every small need from disturbing the whole luggage structure.
For group travel, early shared rules usually work better than constant polite improvisation
Cabin order breaks down when everyone stores items however they want from stop to stop.
Agreeing early on what belongs in the access layer and what must return to the main luggage layer usually saves far more effort later.