Going to Lake Powell I knew it might be challenging to have any decent trees to hang from and I was right. I got to the campsite and all I had was one decent tree to hang from. It was so hot (for me) that I thought I would try hooking up to my Xterra's roof mounted cargo carrier/luggage rack. I had googled and searched for anyone else doing this kind of thing with their Xterra before going and it seemed plausable. As you can tell it worked like a champ:
A close up of my connection method on my Nissan Xterra. I figured that attaching to the base of the support would put the best distribution of horizontal force on it - I wanted the force to be carried by the metal hex screws as much as possible and by the plastic as little as possible. I tried this connection on multiple support posts on my rack and they all seemed equally solid.
The other end of my connection - just a standard tree hugger and carabineer (Note that I use a carabineer as my toggle and hook my whoopee sling into the carabineer, I have never had a problem)
As you can see, it worked great! I got no crazy plastic cracking or creaking sounds, it felt rock solid. I weigh about 205-210 and it held me just fine all night.
So if any of you have an Xterra, give it a try. I see no problems hooking up a hammock from each roof support.