I saw a wooden design of a phone amplifier in a book called “Build stuff with wood”. I decided to make a 3D printable version. If you don't use the profiles, I recommend you manually add the supports only at the beginning and the end of the “tube”. No need to put support inside, your printer can bridge it. It is designed for phones with the speaker on the right. The phone slot is 80mm wide.

Don't expect too much off this, the amplification level is not incredible, but it makes the music a little louder and clearer. It can also serve as an oversize phone holder!