I use a 5 pin Bosch style relay. Terminal 86 and 30 are wired to switched power. 87a is connected to the Boyer power wire. Terminal 85 is wired to the kill button which grounds to the handle bar when pushed. If using the correct 35601 switch, only one side need be wired. In this configuration, when the key is on the Boyer sees power through the "normally closed" terminal 87a. When the kill button is pressed, the relay is energized by providing ground to the coil and 87a is opened. I put a blank insulated terminal end on 87 as it is energized when the kill button is pressed.
These are available in "mini" version as well.