Originally Posted by LarDasse74

There is another type of adapter that is made that is a piece of steel tubing with a lower diameter that fits into a steerer tube, and an upper diameter that is the same as a steerer tube so you can put a quill stem down inside. If you have one of these and the upper outside diameter is correct for clamping a threadless stem, you could put a star-nut from a threadless headset into the top of it to mount your e-phone. But then you will never be able to remove the adapter unless you drill out the star nut.

Edit: Here is an example of this type of adapter, but this one is the wrong size:
http://www.bonthronebikes.co.uk/513-471916