Now that we have the electronics and code figured out, let's built a box to put it all in. Inspired by John Park's recent guides, we'll use cardboard.
We can start with this Instructable on making custom cardboard shipping boxes. We need a much smaller box, but it works well enough. After some quick measurements of the arcade switches and Circuit Playground Express we can see that a 6cm x 9cm x 4cm box will fit everything, snuggly but well enough.
For such a small box, the small flaps in the middle are overkill, and actually get in the way. They can be removed. Once measured, cut, and the bend lines (shown as dashed lines in the template) are partially cut, we're ready to add the required openings. You can see the holes marked and poked through for the Makedo Scru connectors.
Flipping it over, you can mark center lines on the top and bottom sections that will be used to position the Circuit Playground Express and the arcade buttons. 30mm dia. circles are marked for the buttons.
You can mount the Circuit Playground Express using double sided tape and a scrap piece of cardboard to raise it so as to give some room for the alligator clips.
The buttons get pushed through their holes carefully; their snap-in wings should pop out and secure them in place.
The button connections have been bend to give a bit more clearance between the buttons and the Circuit Playground.
Now it's time to connect the alligator clips and carefully fold the box together. Put the Scrus in the two ends and check that it works.
If it doesn't work, open it again and check/fix the connections. Once it works put in the final Scru.
A set of plastic eyes completes it. It won't win any engineering awards, but it works.
If you want to use it with sound rather than the USB keycodes, you'll need to make the box a bit bigger so you can fit a LiPo battery inside. Be sure to place the batter in a way that you can easily disconnect the cable from the Circuit Playground Express so that it can be recharged.