I have a friend who is a guitar teacher, studio engineer and all-round musician and who also posts tutorial videos on Youtube (check out his channel, Play Your Guitar - https://www.youtube.com/channel/UCbrV1Dg2YVKQa8ibxvnAMTg ). He asked me to design a mount for his iPhone to attach to a guitar neck so he could take photos looking down the neck, so he could demonstrate picking techniques etc, and here's what I came up with. He's in the process of field-testing the prototype at the moment and hopefully should come up with some great footage, but here's the mount for your use too.
The mount is sized for the neck of a traditional electric guitar such as a Fender Strat or a Gibson Les Paul, I'm not sure if it would fit onto the shredmonsters like Ibanez and so on and I'm not sure if it would fit onto an acoustic. To attach it slip it over the neck around the nut and pull it down the neck until it wedges; it should stop at about the 5th/6th fret. Obviously it's going to restrict neck access so it'll be more for playing lead than for rhythm and you may not be able to throw your guitar around as much as you usually do or the phone will go flying across the room.
Enjoy, and let me know how you get on!