All this tries came out from my worn micro USB connector (it was charging some times, and other not).

Therefore I use this external coil setup on an old cover adapted for the use.
It works well, better that the sandwich setup (the sandwich/spacer solution has some problems with the camera open/close activation, tunnel view to the camera, and I was obligated to add a magnet to enable MMC recognition).

Of course the coil is visible because I extracted it for demonstration purposes.