Finally I got this:
Wirefree MP3 door chime kit
This was the only one I was able to get in Japan, and it worked well, at least at home. I placed the sound unit in the frunk upside down so the speaker is facing up. I was able to hear the melody sound from inside the car.
The sound volume was pretty loud but I haven't tested it in the crowd so will report later. Per my test, placing the unit in the frunk, the sound seems a bit weak.
Wirefree MP3 door chime kit
This was the only one I was able to get in Japan, and it worked well, at least at home. I placed the sound unit in the frunk upside down so the speaker is facing up. I was able to hear the melody sound from inside the car.
The sound volume was pretty loud but I haven't tested it in the crowd so will report later. Per my test, placing the unit in the frunk, the sound seems a bit weak.