They make covers for pickup truck beds. Some are flush with the top of the truck bed and some flush with the roof of the cabin, which is called a camper top. That said, I do believe that rule is no longer in effect. IIRC they made it so that unaltered pickups under 5,000 lbs can register as passenger vehicles. Prior to that if you didn't alter your truck it had to be registered as a commercial vehicle and that restricted which roads you can drive and park on. I could be wrong on this, but I remember something about this in the news 20-ish years ago.