Okay, so this company called MakerBot has a file to print out parts for a cassette-shaped MP3 player, assuming you have the wherewithal to purchase one of their Replicator 3D printers. If you do not suffer from such a surfeit of simoleons, then you can always just order one from them completed for only $40 ish. No word on the official site on how much this device holds, presumably as much as a cheap player would, though.
Now, this is all very nice, cute even. But I have to ask, why stop there? If you really want to recapture mix-tape magic, why not create an MP3 player that works in a cassette player?
I mean, it seems simple enough; a pressure switch that turns the player on when the play button on the cassette player pushes the head and/or capstan into place, and a device to transfer the music from the player to the head (a device which has existed for years). Easy-peasy. I'd buy one, maybe. Especially if it held hundreds of songs. And it'd be great for anyone who still has a cassette player in their car, eliminating the wires (or wireless and interference) of current systems of playing your MP3s on a car stereo.
Oh wait. I really should research before I spout off. This appears to be similar to what I've talking about.