This passive phone speaker/cradle is cheap, effective and easy to make. It also sounds better than most of the small, solid passive phone speakers, probably because of its large reflecting surface.
The speaker/cradle should work with most phones, iPhones included.
Step 1: The Basic Idea
The first picture shows how the 2 liter soda bottle is cut, and the second one the result.
Serendipitously the bottom of the phone rests nicely on the bumps at the bottom of the bottle, and the curved back reflects the sound effectively.
The 2 liter Fanta bottle in the last picture works perfectly for this project because of its straight sides. Other 2 liter bottles should also work, but the marking and cutting will be more difficult if the sides are curved.
Step 2: Marking Out the Bottle
First draw a line around the bottle 160 mm from the bottom. I fitted a piece of A4 paper around the bottle to guide the pen (first picture). Repeat the process, creating a line 45 mm from the bottom of the bottle (second picture).
Then cut off the top part of the bottle along the upper line with a pair of scissors. DON'T cut off the bottom part.
Now put your phone into the bottle (third picture) so that rests nicely with its bottom against two of the bumps, and its back against the back of the bottle -- thus the position it will be used in later on.
Put a ruler across the top of the bottle's top edges, and mark the left and right hand side where the downward cuts will be. I made my marks a little bit less than halfway across from the phone, as we don't want to focus the sound too much.
Turn the bottle upside down, and with a try-square draw a line down the side of the bottle using the mark you've made in the previous step (fourth picture). Repeat on the other side.
Before reaching for the scissors, first find a suitable round object to round over the corners of the cutlines (last two pictures). In my case the cap of the lens I was using to take these photos was the perfect size.
Step 3: The Final Cut
With a pair of sharp scissors, cut neatly along the lines (first picture), and you should be left with the final product (second picture).
Step 4: Finished!
All that is left to do is to put your phone into its new cradle, and to enjoy the much better sound it now has.