I plan to mount a completely plastic device on my Phantom 3 directly beneath and beside the Compass. Apparently a digital (or analog) compass is not affected by plastics unless they contain metal particles. (Unless the plastic is extremely thick) since hydrocarbons alone do not block magnetic fields. Bottom line is should have no affect. I think you could wrap a 1 inch thick plastic clamp around the compass and it would have no effect. The compass is actually already housed INSIDE a plastic housing. So I'm just adding a bit more plastic around it. But, just for extra measure, does anyone have any specific experience with mounting a plastic device under the compass of a P3 such as a leg extender or accessory mount? So far it does not seem to have affected the calibration or MOD values.