I made this smartphone amplifier in a gramophone's style, and it's working great!
The horn is supporting the base and is standing with the weight of the smartphone. Note when removing the smartphone to be careful because the horn is making the whole thing "falling" on the side
I had to slice the horn in 3 parts because my printer's bed is only 6 by 6 inches / 15 by 15 cm
If you have a bigger bed you can try to print the "gramosmartphone_horn_whole.stl" for a nicer result.
If not, print both "gramosmartphone_horn (Cut 1)refined.stl" and "gramosmartphone_horn (Cut 2).stl" and stick them (I used glue for ABS)
Also print the base for smartphone and stick it. I had to slice it with Cura because the new Repetier Slic3r did not well
I printed with a 6mm brim
My printer only prints in 0.3 without fan, the result could be more precise
You might have to sand a little the inside of the base, to be closer to your smartphone, depending on the precision of your printer