All of the 3D printing files can be downloaded directly from Thingiverse. Fusion360 files are also available for the individual models if you want to remix them.
To allow your Feather to live its best life as an 8-bit synth, you need to 3D print some keyboard keys. These keys are sized to snuggly fit over the circular top on these tactile buttons. They also print with no supports and are pretty cute.
Since the Circuit Playground Bluefruit is acting as the amplifier for the synth, you might as well print a little amp-inspired case. The case is snap-fit and prints without supports. The front of the case has a pattern that make it look like an amp speaker grill. The CPB and STEMMA Speaker attach to the front via some M3 and M2.5 sized holes that allow for screws of the same size.
Depending on whether you're using the PCB or perfboard for the Feather portion of the circuit will determine your enclosure. 3D printed options are available for both, along with their Fusion360 files.
For the perfboard version, a shallow open box style enclosure is available with lips on the edges that keep it nice and secure.
For the PCB, a base was created that allows for the PCB to sit on top with stand-offs. Much like the amp enclosure, it allows for screws or standoffs to be attached.