Thanks to you both for your responses.
There is a rack that is made to work with the Ramboxes. It attaches to the cargo rails. It uses the top channel, leaving the side channel open, so I'm looking at tonneaus that can hook into the side channels.
Here's the link to that rack--
Vantech P3510s Dodge Rambox Silver Aluminum Pickup Truck Rack - RackWarehouse.com
It's only rated at 150 lbs, but I only need it for two Kayaks. The height is adjustable and it looks pretty good. Plus, it's easy to remove when not traveling with our toys. Now, to find the perfect tonneau cover.
Seems to me the Backflip tonneau cover will fit, like that
rack, on those rails. Since I don't imagine you want that rack
hanging out there all the time, maybe you should just swap
the rack with a backflip as needed???