In theory you could install a roof box or a cargo carrier without crossbars. You would be able to install one over a platform rack, or even over a bare roof as long as you have straps to hold the box securely over your roof. However, cross bars are the best and easiest way to have a roof box over your vehicle. The reason being is that most cross bar manufacturers also make roof cargo carriers that perfectly fit their cross bars shapes and design. Therefore, if you plan to purchase a cargo carrier, then it’s a good idea to buy a set of cross bars from the same brand. You will notice at the bottom of these roof boxes, there are two horizontal channels that seamlessly fit the cross bars from the same brand. Resulting in a safe, secure and rattle free transportation of the box itself.