Securing a cantilevered awning to the roof like you've drawn without modifying the roof would be a challenge, there would be a lot of leverage on the attachment to the roof. You'd want a roof rack for that, which wold defeat the whole purpose of a "no rack awning" .I understand the need for the lines I just would want more of a compact space if used for tailgating. I like what was suggested about the poles connected to the bumper. Then the lines could be dropped vertically making less of a tripping issue and able to use in tighter spaces. Just a thought.
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About a shorter awning for tailgating, the way I've done the design it can be installed at multiple lengths so a smaller size when needed is easy. Just roll up the front of the awning and secure it using the middle grommet. More grommets for different lengths could easily be added.