So, I’ve reworked the PCB into what I expect to be the final revision. I eventually decided to add the coin cell battery holder on the PCB itself after all, because it would be a little fiddly to add a holder for two AAs on the back. I’d have to either mess around with glueing it on the back (and making sure that the PCB matches the size), or figure out some kind of screw mounting. Since I don’t have a proper glue gun yet and project box making is something I still have to learn, I figured this was a decent enough solution after all on the level that I’m currently at.
The front of the PCB 
The back of the PCB
As you can see above, the result of adding the battery holder is that the entire PCB is now a little wider and less high, as I shuffled components around slightly. Another thing I noticed was that the pins of the ISP had no significant marking for orientation. I had a look at how it was marked on my USB programmer and noticed that this was done on the back (probably to save space), and that seemed like a good practice to follow. All in all, I’m quite pleased with this final design. Before I do go ahead and order it, I do want to test it out on a breadboard; after all, you never know when you’ve made some silly design error.
So, next step will be to order the components. While waiting for them to arrive, I can rewrite the code already to fit the ATtiny84, so that I can test it as soon as everything is in.