The concrete shape I need was tricky to make; it needed to look clean and smooth on top of the shelf, but hollow underneath to accommodate a wireless charger and manage the cables.  I used an instant concrete mix called Quikrete, and made a lubricated plywood mold. I suspended a block into the mold to create a cavity, and used wood biscuits to create openings for hanging hardware. 

 I got the cast out of the mold pretty cleanly, but it cracked in half when I took out the space to make the cavity. There's definitely a way to make an intact cast, but I decided to repair the original with black dyed epoxy. I actually like the marbled look.

I picked up a cheap Qi wireless charger and ripped out the board. A few dots of epoxy fixed it inside the shelf. I couldn't find a standalone circuit to buy, but I sure there are some wholesale options like Alibaba. The one I got charges very fast through about 0.5 cm of concrete; I use an iPhone 8 with a thick plastic case and can get from 20% to full charge in about an hour.