It’s the handiwork of a modder going by the name of techknott, and it’s his second such portable GameCube mod. (I don’t know where the first one is, sorry.) You’ve got an LCD screen, integrated controller, and a headphone jack.
You simply pop the disc in and you’re golden.
See, if I had any engineering talent to speak of I’d be doing stuff like this all day long. “AS SEEN ON CRUCNHGEAR” and the like.
Alas, I spend my days reading soccer tactics instead. Shame, really.