Here is a DIY design & build of a small (300 class, 9" props) compact foldable quadcopter you can take in a backpack, weighing less than a kilogram (including a Drotek RTK GPS, CUAV V3 pro pixhawk autopilot , telemetry radio)
It can fly a good 30 minutes with a 3-axis Gopro type gimbal (subject of part2). The frame is foldable (with and without props attached).
It flies of course with Arducopter firmware V3.4.6.
For those inetrested , I had made a previous review of the CUAV V3 pro (Pixhack) autopilot :