A hard box is the rigid cargo compartment that turns your bike's flatbed into a proper carrier: solid walls, a floor, and usually a lid, in aluminium, marine plywood, or recycled plastic. Unlike a soft bag it holds its shape when empty, locks shut, and stands up to weather and rough loads, which is why most owners fit one as their first real upgrade.
What you choose depends on what you carry. Plywood and aluminium boxes from makers like Larry vs Harry and Badger Bikes are built for kids, with double and triple seating, cushions, and belt mounts. Recycled-plastic boxes from MORE Cargobike are lighter and lock, better for tools or delivery runs. Prices run from around 50 EUR for a simple stackable insert to 1,500 EUR for a premium locking box, so there's a fit for a school run and for a working bike alike.
Two things to check before you buy: whether the box needs a matching base plate to mount, and whether it's cut for your exact platform (most here are Bullitt-specific). Carrying a dog? See dog boxes. Kitting out for kids? Start with family and kids.
