How to create an arcade type button with a plastic bottle cap.
As you see, the most important and unusual part, is the O'ring. it can be of any size, as far as:
1) it has top and bottom inner walls. this will hold the washer on the bottom and the plastic cap in the top, as you see here.
2) it must be of rubber, to have the flexibility to adapt to the form of the bottle cap and the washer.
When you already have it, you can buy a washer that has the same or slighter larger size of the O'ring
Insert the washer inside the O'ring, adjust to one of the edges inside of it
Solder the switch pushbutton in the protoboard with a couple of wires and glue it to the washer