[A930Rocket]

My first inclination was to put the boxspring on first and the mattress on top. But thinking about it more, would it make sense to put the boxspring on top, to help prevent the mattress from bending up while driving?

As mentioned, spread the bars as far forward and aft as possible. Strap the mattresses together and run ropes fore and aft. When we kayaked, there was a type of knot we would use to keep the kayaks on side side and front to back. In the front, you could probably find a tow hook or something underneath the front bumper for your rope.