ECF, this might work for your "shelter".

take some 2x2 and cut them to fit the stake holes in your bed rails. IN THE FRONT next to the cab, take another (2x2 maybe) and nail or screw to the stake hole boards. forming a brace as it were from one side of the truck to the other. Then you drape the tarp over and then UNDER this "Brace" and lower it into the stake holes (you could also use some Velcro to attach the tarp to the brace)and this would help any moisture to roll off the front and not get into the bed. Then do basically the same thing for the back of the bed but with taller uprights. This would in effect give you a lean to in the back of your truck without having to use any para cord. You could even make side "braces" to go from the front to the back on each side as well. And if your tarp was long enough you could let it hang down below the tailgate and provide a flap.

Hope my explanation is clear enough for you to understand. I have a hard time getting my ideas across some.