Sharp-eyed dog walkers along the San Francisco Bay waterfront may have spotted a strange-looking plane zipping overhead recently that that looked strikingly like the U.S. stealth drone captured by Iran in December. A few key differences: The flying wing seen over Berkeley is a fraction of the size of the CIA’s waylaid aircraft. And it’s made of plastic foam. But in some ways it’s just like a real spy plane. The 4 1/2-foot-wide aircraft, built by software engineers Mark Harrison and Andreas Oesterer in their spare time, can fly itself to specified GPS coordinates and altitudes without any help from a pilot on the ground. A tiny video camera mounted on the front can send a live video feed to a set of goggles for the drone’s view of the world below. “It’s just like flying without all the trouble of having to be up in the air,’’ Harrison said. Thousands of hobbyists are taking part in what has become a global do-it-yourself drone subculture, a pastime that’s thriving as the Federal Aviation Administration seeks to make the skies friendlier to unmanned aircraft of all sizes.