No matter what type of cargo box you have, it’s recommended to distribute half of the weight over the top and in-between the two crossbars. Then, a quarter of the weight should be in the front, and the other quarter should be in the rear of the bars.
When deciding what to put in your cargo box, make sure you know the weight capacity of your specific cargo box. When prepping your gear to pack in the cargo box, consider wrapping everything in garbage bags for an extra layer of water protection.
When your cargo box is fully loaded up, double-check and then triple-check it is securely closed and locked. For extra security, consider tying the cargo box down with ratchet straps.