I used scraps of copper pipe to make the sound tube and the clip, the clip was cut from a short length of pipe that was split longitudinally using tin snips and opened out and hammered flat.
The end of the tube that clips over the speaker slot on the phone is flatted into an oval shape to cover the mice better. the clip is a flat strip of copper about 80mm long and about 20mm wide. i folded a small angle at one end and shaped it to match the pipe before soldering the 2 pieces together. the clip was then formed with pliers so the mouth of the tube sits directly over the speaker slot in the case.
The megaphone part is a plastic drinks bottle cut down to your desired size, experiment with different lengths and shapes of cone as it does change the tone. i also tried the metal cone from a bicycle horn but it did not perform as well as the plastic bottle. The bottle cap is drilled out to 14mm and the 15mm pipe is just a push fit. the screw cap also also you to experiment with different bottles without having to glue and unglued as you try different cone lengths and shapes.
I use a fake leather flip case on my phone so the clip can grip it without damaging the touch screen on other types of case the clip would have to be padded.
I have uploaded a video, but i had to use my old camera so the quality is not great and the sound is allot more tinny than it really is.
Thanks for looking and i hope you like the idea.