Here's a case that works with the Feather-compatible Argon and Xenon (Boron compatibility hasn't been tested).

Argon: There is a cutout in the back of the case for the Argon's Wifi antenna (fish it through and connect before enclosing).

Magnetic cover: There is a cover with slots cutout for a badge magnet, so that the headers can be exposed on the bottom for free-wiring.

Magnets: A compatible badge magnet can be found here, on Amazon. Put the badge magnet in, and place the retainer bar into the two slots (flip for compatibility between different badge magnet styles).

This is a derivative of this case, if more pictures help (this one does not support a battery on the side): Adafruit Feather Case (Badge Magnet + Battery Support).