Commercial store bought are pretty much worthless for your discribed use. Consider building your own with many tie points and fastened securely to the top box rail better then the system used by the rail industy, which is an expansion system in the pocket. I wouldn't tie a dog to the different rail systems I've had on my trucks and feel very comfortable the load would stay or be there when I arrive at my desination . . . . Mostly for looks then practical use of secureing a load IMO.

Getting any rain now ?