Oooh, ouch.; not pretty. I'm not sure about using duct tape, but, I would suggest with something large that you not only tie the mattress onto the racks (or through the doors onto the roof) but, also fore and aft (i.e., tie the front of the mattress down to the nose of the car and the back to the rear bumper). One of the real problems can be the front lifting and loading up your straps to a point where they snap. I'll look okay at a stop, but, at speed, things may get squirrely. The more the object conforms the the shape of the car and stays close to the car, the better off you'll be. I've had 2 kayaks, full size couches, etc. on my Protege (not P5) at highway speeds, no problem, but, if things slip... it can go bad fast.
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