While I would never think of doing this, I am happy to see that Area 53 firearms offers a Challenge Coin lower receiver. Designed to fit any standard 1.75″ challenge coin, the lower receiver’s right face is countersunk to accept a coin. The install is permanent (requires epoxy to attach the coin), but would create a novel gift. Personally, I would like to see it countersunk on both sides of the receiver so one could mount two coins and see the front and back.
The Challenge Coin receiver is available now for $176 at Area53.com