7/8” Round roof cargo rails cargo box?

Hey guys! I recently purchased a truck camper from Capri Campers. I haven’t mounted the rails yet, but they gave me two roof rails for bolting on the roof to mount a roof cargo box. The rails sit on mounts 3 inches above the roof and I could run them parallel or perpendicular to the truck. The rails are 7/8 inch round stainless steel. I’m trying to figure out how to mount a roof box (likely Thule or Yakima). I can’t figure out if it’s possible to mount the box directly to the round cargo rails and, if I have to use a cross member, how even that would attach to the rails. Alternatively I could ditch the rails and use rails from a company like Swagman. Just looking to hear some other experiences and methods and apologize if this topic has already been beaten to death.