Pull back the plastic trim cap and you will find a series of molded in holes. Their kit comes with u-nuts to put in those spots and a 1/4” drill jig to locate the mounting holes you need to drill through the plastic cap. (You don’t drill into the box). The bolts pass through the new holes in the plastic box rail cap and thread into the u-nuts that snap into the factory spots in the box. The box was designed for the kit-included mounting nuts. If you have a Carbon Pro, they are there, just peek under the trim cap.