Last night I installed three "admin" buttons. I bought 3 Happ buttons from ebay to use as power, coin, and escape buttons. Some people also install a pause button. I drilled holes underneath the control panel so that these buttons won't be visible when you look at the cabinet but you can reach them while you're playing.
I drilled a 1 3/8 inch recess and a 1 1/8 inch hole to install the buttons. These are the standard measurements for the standard Happ buttons. When I get the computer in there, I will discuss how to hook up these buttons to the software.