Going to suggest just going to Harbor Freight or your local hardware and buying a normal, heavy duty tarp, if you really need to cover everything.
I say that because, unless you're doing it daily as a commercial driver, law enforcement generally doesn't bother people hauling loads of rock or wood in the back of their pickup truck, mostly because it really isn't going to go anywhere anyway, and because those laws really only apply to commercial vehicles.
I see people doing what you intend to do DAILY without a tarp over the load.
That said, if you were hauling things lightweight or that could catch wind and fly out (like shingles or other home improvement debris), then it would be a good idea to cover it to prevent it from flying out.