Origaudio Universal Rock-it 3.0 Portable Vibration Speaker with Rechargeable Battery 3.5mm - Black
I have attached this to almost everything I can think of,  cardboard, plastic, sheet metal, beer cans, hollow core door.  It does not produce the best sound but it is good enough and loud enough to hear plainly.  It has always been a crowd pleaser.  My friends have been amazed at all the thinks that will produce sound.