A PCB was created for this project and the Eagle files are available below. If PCBs aren't your thing though, don't worry- there are instructions below on how to solder up a version using perfboard. Since the circuit is straightforward, either option is great.
For the PCB assembly, place the buttons into their spots on the board and solder each of their four points on the back.
Then, solder some Feather headers into the header holes. And with that you have a fully assembled board.
You can download the Eagle files for the PCB below.
The PCB was designed with the round tactile button switches in mind. They're also used for perfboard version and the 3D printed piano keys are sized for their round toppers.
For the perfboard assembly, you need two pieces to start. Begin by placing 6 of the buttons on the first piece. The buttons should be placed 5 holes apart to have the spacing work for the 3D printed keys.
Cut pieces of wire for the ground and input pins for each button. The ground connections will daisy chain between each button, so they should be just long enough to reach the next button. The input wires will need to be long enough to reach where the Feather will eventually live on the far right side of the assembly.
Repeat this process for the second piece of perfboard for five of the buttons.
To make the connections on this type of perfboard, you'll need to bridge the individual solder points. For example, the input wire for the first button should have some solder that connects the two points.
Next, cut the right edge off on the first piece and left edge off on the second piece. This will allow placement of a button, which will eventually be F#, in the middle. Place this button on the edge of the first piece, so that it's hanging off. Then run a short piece of wire to connect it to the second piece.
Then, we're going to cut another section: the last column of five rows on the right hand side of the second piece. Then solder on Feather headers as shown in the picture.
With the headers in place, connect the button wires to the Feather. Run the input wires to the appropriate I/O pin and then bridge the connections. Do the same with the ground signal.
Finally, to add an on/off switch. Place it on the board and then run a wire from GND to one lead on the switch and then a wire from EN to the middle lead. Connecting EN to GND allows for built-in on/off control.
And there you have a perfboard equivalent to the PCB.