Reuse a paper cup. Preferably from your awesome local coffee house.
Begin by working in a square shaped bottom...
Pinch and fold down to the bottom of the cup.....
Like so. (Note the creases down the inside)
Make phone shaped hole in top. Slightly smaller, as you want a snug fit... should be able to hold the phone in place.
Put on some sweet tunes.
You really have to try this to hear how much better the sound is! Enjoy!