MP3 Cassette Solution

Step 3 Open them up!

Here we can see the innards of the adapter and the naked mp3 player. In my case I stripped out all of the plastic cogs to make room for the player. A miracle! The wafer thin player fit snugly between the plastic pegs inside the adapter. (Some shaving of these pegs with a sharp knife helped - and some pegs had to be cut out alltogether.)