Sorry, I failed to take any photos during the build. I will do a post on the clock structure/ interior the next time I build one. (which should be soon). Most of the box types have a similar structure.
My first batches of clock brains used a trinket Pro 5 volt which is similar to the nano. At some point they were either on backorder or rumored to be discontinued so I redesigned for the trinket m0 which is a buck cheaper, and has sufficient I/O for the job. It is otherwise overkill in terms of capabilities.
But, since I use the same board (with clock components deleted) for the led lanterns and other (often higher count or trickier animations) LED matrix projects, it made sense to use it. The shift register was not a hard addition.