So I found this thing on Amazon for about $8.
It’s a wireless charger. Micro-usb cable plugs into one end.
Looks like it was yanked from unsold product, or perhaps faulty product that was repaired. The first one they sent me worked, briefly. But it seemed like it had a bad solder connection or something, so I got a replacement that worked.
It seems to need a gap of maybe 2-3 mm between the phone and the coil. A piece of cardboard would probably do.
But if you’ve got a 3D printer, you can download this thing I designed in OpenSCAD and print yaself a case.
The phone needs to be centered pretty well over the coil, so I made a top with a cutout to fit the Samsung S6 that I use to play Clash Royale.
Works pretty well. Once I get the new 3D printer running, I should be able to run flexible filament through it and get a nice flexi-holder that should grip the phone a bit better.