Here are photos from the assembly.  If anyone is interested in replicating this I can write up more detailed instructions.

Assembled ULN2003 stepper wing with IR beam-break (for detecting pellets) and 4-pin circular connector attached (for connecting to light-up button)

3D parts, assembled button, and plant:

Installed stepper into 3D housing:
Dispensing disk attaches to stepper with an M3  brass insert and set screw:

Feather board goes here:


ULN2003 stepper wing goes on top:

Front view showing photobeam clicked into the 3D housing (there are screw holes but they aren't really necessary):

Final assembly