An Emergency stop button from an simple switch. 

To assemble it, you nee 4X 22mm m5 screws with fitting nuts. I designed the hholes, so you can insert them with an soldering iron. You need to cut the nut, from the switch smaler, so it does fit. 

Sadly the button is closed, when the switch is down. So in an emergency it is you have to press it and then it sticks out 2 mm more.