On the first MPGuino I made, I had wires running out for the switches, and I had the switches superglued to a piece of plastic. They held great, the rest of the board was a mess though, and I ended up getting a kit version to keep it clean, so I didn't need the switches.
I would just drill small holes in the top (enough for the button part to stick through), and glue the square part to the top of the box. Superglue, or hot glue should work fine for it. Then, just connect the wires from the switch to wherever they went on the MPGuino.
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