My first draft is a slightly more fancy version of the "tarp and some poles" awning.
The car-side support is an expanding fiberglass painter's pole that is clamped to the top of my roof-basket with U-bolts. The tarp is fixed to this pole with strapping (into grommets in the tarp, including some added grommets where the originals didn't line up conveniently). To deploy I extend the pole and tighten the adjuster, then add the two expanding poles and some guy lines.
To store, I just collapse the painter's pole, fold the tarp so that it is only as wide as the shortened pole, then roll it up tight and fasten it to the pole and roof basket with a few wide velcro straps.
I did it this way because the roofline of the Subaru didn't give me much length for a tarp. Being able to extend the painter's pole allows me to use a full 8x10 tarp with only two poles and two guy lines. (The third in these pictures is to keep the leading edge pulled down into the evening wind coming off the bay)
Incidently, my "tent poles" are some sort of 4-stage collapsible aluminum poles for radio antennae or something, they'll extend to more than 25' long! (though not rigid enough to use this way at those lengths, very handy for getting kites and frisbees out of trees, ask me how I know). I got 'em from a local industrial surplus place here in SoCal.