This is intended as an accessory for the Bicycle BoosterPack (Project seen here: http://bit.ly/28XhHxg). Originally I recommended buying a handlebar button assembly from Amazon but they have since tripled their price to ~$18. This model will enable you to get the same functionality for the price of the switch & a screw ~$5.
There are two different sizes provided depending on where you want to mount it. See image for standard bar sizes. Most folks will need the .875" diameter size as I did.
The 'Bike Handlebar Button' could easily be used for other purposes, such as a horn or light.
(1) Normally Open (for BoosterPack) Momentary Push Button Switch: http://amzn.to/299XqS8
(1) #6 pan head machine screw, 2" long, with nut. (available @ Lowes, Homedepot, etc)