As I solder more of these PCBs together and assemble cases, I keep learning better ways to do things. I've made some minor changes to the case design which help it print more reliably, as well as to make it easier to remove a PCB if it needs servicing.
I have also experimented with adding a port hole to the rear of the speaker box. I did a lot of reading on speaker port theory and figured I'd give it a go. With a port hole on the rear of the box, bass came out somewhat less distorted, but at the same time certain parts of songs now had horrible distortion at high volume. I decided to go back to the original design. I ended up modifying the speaker grille to allow better airflow and now I'm fully satisfied with the case design.
I've added a picture which shows the latest version of the speaker case. To make it like that it was sanded flat after printing and then smoothed out with an acetone vapor setup which you can take a look at here.
On another note, the battery life of this device is amazing. I had initially wanted 6-8 hours of battery life but I've ended up with much more than that. I've been testing the first unit I built which so far has lasted around 20 hours from a full charge. It's not even telling me the battery is low yet, so I'm not sure how much longer it will last. It seems the class-D amplifier I used is quite efficient.