I'm not there yet but once I am I will post details about the installation procedure and parts used. But do a search on the forum. I'm quite sure there are other forum users who already installed a power switch and have mentioned it. But it is simple: just cut the red wire from the battery connection (on the Phantom, not the battery of course) and solder a adequate 2 position switch (look for it at a car parts shop) to both wire ends. Be sure to insulate the connections and the switch itself with e.g. vulcanizing tape. If the red wire is too short simply use a longer wire of the same gauge instead. If you don't want to cut a big hole in he body for the switch, I would suggest you attach it to the landing gear or some spot like that. You could attach it with screws, a tie wrap, double sided tape or extra strong velcro (3M). The red wire could be fed out of your Phantom by making a small notch in the upper shell, feed the wire out to the switch and feed the other end, coming from the switch back in. It all depends on the chosen location for your switch. Make sure you choose a spot that you can safely reach while the props are spinning.
I would not use a switch with fuse or thermal breaker. The peaks are pretty steep sometimes with these motors and the Lipo's can discharge at a very high current so a fuse could blow and cause a crash.
If you attach and solder everything carefully and make sure everything is well insulated, I don't think adding a switch will introduce an extra single point of failure, as long as it is not fused or thermally protected, so the most simple (but high power) switch will do. Get a good one, pay a few bucks more for it. There is lots of choice.
I will post my findings once I start working on that.