https://www.youtube.com/watch?v=NXjCf5l7qDA

Wow this is interesting.  If you mount gopro hero 4 on top of the mavic battery it kills the satelite.  
If you have one of those 360 camera and mount it on mavic or phantom you can have flying VR unit.

I believe if you cover the top at all, it's going to block the GPS puck from picking up satellites.

Makes sense that long extension wouldn't work.   That would severely change the center of gravity, making it top heavy.
As for GPS, the GoPro electronics is probably scrambling the GPS signals.  These devices are basically high powered computers.