Storage boxes... there are too many to choose from. But I opted for the aluminium ones from Zarges. I like them a lot, but it was a bit of a puzzle to find a good mounting construction.
Some people adviced me to just secure it with straps, others told me to bolt them to the roofrack. I didn't like either option. The straps would make it way too easy to steal the boxes. The bolts through the bottom of the boxes would make them less resistant to water, would prevent me from easily reconfiguring them or taking them off and I would need to lock each one seperately.
After a bit of thinking I came up with a bracket that was placed on top of the boxes. This way it was holding the boxes in place and locking them at the same time. And it was easy to use since it only used two lockable knobs.