I wanted to retain the same "always offline" and "physical" aspects of the experience. I used NFC labels hidden under the cover art label on old (edit: not SD) CF cards because I had a bunch of old ones around and they were all standard size and not easy to swallow. An internal microSD that held all the files needed. Plugging the cartridge in the fake socket which was actually just a hidden power on switch would trigger the playing of a specific file. It's a tad more maintenance heavy but much quicker to pull of.