Parts arrived from Adafruit:

Since there are few connections ( 1 pin + gnd + batt for the WS2812 LEDs, 1 pin + gnd for the button, 1 pin + vcc for the buzzer), I opted to tie everything directly to the header pin holes on the Feather board. With the help of some double-sided tape this is already a the basic device:

I'll be cramming this into a 3D printed shell for now; later versions will consist of a compact single custom PCB.