Hypnotic concepts came up with this simple-genius handlebar mount that allows you to mount just about anything.  See examples above of speaker mount, on/off switch, even a ram ball mount that you can attach a phone mount and USB charger.

Easy install with the provided bolt and spacer.

 Comes with a pair of brackets to mount on left and right side, new stainless bolt and spacer included.  Accessories and speakers not included.