I bought a Curtis receiver hitch for the 2014+ models off CL for $25, planning to modify it to fit my 2007 Justa. After removing the rear bumper cap, I wondered why I needed a second steel tube going across the back of the car, since the bumper support is already there. So I made this myself out of 1.5" square, 1/8" wall mild steel tubing. I hope the pictures are self explanatory.

 

 

 

You raise a good point, one I thought of myself. But I reasoned that the Curtis design would likely impede the crumpling much more than my design. And, I've heard of the Curtis and similar styled hitches bending with a cargo carrier in the back. I think mine is stronger, adds less weight, and is safer in a rear impact. But I, like Curtis (I would assume), haven't done any crash tests to verify my claims. As it is, I wouldn't hesitate to haul 800 lbs. I have put custom hitches on a lot of stuff, including this 1974 Civic that I converted to AWD. With a similar hitch on this car, I hauled a Cummins 6BT engine and trans, and a golf cart another time.

Is there such thing as a 2x4 trailer? Probably, I suppose, but maybe you meant 4x4?