I tried hanging from my car this past weekend, wound up sleeping in the car : (
I was in a blow down area and there was only one substantial tree so I thought I could tie the other end to my Yakima roof racks. The rack popped right off of my car and did some damage to the windshield frame. Won't be trying that again!
I've always feared the those exact results on my vehicles. But I would not hesitate to run a cord from upside down "V" 2x4s ( or even a single 2x4) DOWN to my class 3 trailer hitch.
Apparently, signature that I used from 2006 no longer tolerated so now deleted.