This is a very simple mechanism you can use to drop objects from mid-air with a DJI Phantom drone. It is very simple to use; all you have to do is attach your package with some kind of hook (see custom section), and then fly to your destination. Once there fly the drone side-to-side by moving the right stick back and forth horizontally. This motion should cause the hook to slide downward until it reaches the bottom, then it's bombs away!
You can check out a video of it in action at https://youtu.be/2jpCVHGBHiE
Clean up with a small file and then attach with hot glue and 2 zip-ties. Just make sure that the surface that the hook slides on is smooth.
This is my design, NOT idea
I found a similar product on http://www.gimbal-guard.com/ while looking for metal gimbal guard. I thought it was a great idea but I would never pay $44.90 for a piece of plastic. I jumped on TinkerCAD and 15 minutes later I had a 40 cent drop mechanism.
This is the hook I made, it's just a bent piece of coat hanger.