I think, the 820 cover will do fine. As far as I remember, it outputs 5V. The coil is removable easily (under heat) and very thin. It also comes with something like a shield, I guess*. I tried this before for another phone.
I was always going to try this when I get my Jolla. 'In picking' currently, I can report the result when it arrives.

*<1mm thick, flexible but quite heavy for its volume. Larger than the coil. Can someone tell what this is for? Is it to block the rays going beyond the coil?
btw: Could the electromagnetic field harm any of the other components? I think the field must be pretty strong to charge the battery. What happened, if AC was generated in other parts?