Artists build a bandshell out of old cars, doors and junk in San Francisco. “It took four months to construct the piece: Sixty-five car hoods were plucked from Bay Area junkyards; the steel beams were extracted from a closed winery in Napa; the French doors that form the stage were lifted from a shuttered school near Stanford. The group set up a Web site to book stage times for anyone who wanted to perform, at no cost. As of last week, all the slots had been booked through the summer.”