Just electroless plate the interior of something like this [0]? Or use conductive paint if the polyurethane doesn't take an electroless well. Don't know what OEM Nillkin uses but I'm sure the factory has iPhone cases as well.

[0] https://www.amazon.com/Nillkin-Samsung-Leather-Magnetic-Prot...

Edit: Oh, missed the "toggle on/off without removing the device." Nah, that's not a thing that's going to work. Even with the flap open you'll be attenuating enough RF through the back that either you won't have a decent connection, or your battery life will be crap.