Do I need to add anything to secure cargo on these bars?

These bars provide the foundation for roof loading — you'll need compatible crossbar-mounted racks, baskets, or a roof bag depending on what you're carrying. The bars themselves include what you need to attach to your factory roof rails.

Will these bars work with aftermarket roof boxes or bike racks?

Yes. Standard roof accessories designed for cross bars will work here. Make sure whatever you're attaching has compatible mounting feet for cross bar systems.

Are there any fasteners included to install these to my roof rails?

Hardware for roof rail attachment is not specified — verify against your vehicle's factory rails before committing to mounting. You may need existing OEM hardware or separate fasteners from a roof rack kit.