This is a mini amp I made from parts purchased on Amazon. The total came to about $43 shipped, and there are enough parts to make 2 amps.
Here's the final product with instructions and part links:
And here's the basic hookup diagram.
I highly recommend a bigger speaker, however.
Be sure to get the battery negative connection correct at the TRS "Ring" so your amp only turns on when a plug is inserted!
Here's another diagram:
Speaker connections:
Battery Positive only
Glue in your battery box:
This is the amp and speaker cabinet I made, but pick your favorite little box that fits all the parts:
Glue in your speaker: