This time the assembly took me longer than usual, for two reasons.
First of all, I couldn't get the QFN chip soldered properly, somehow getting shorts or bad connections, despite having nice long pads and using a hot air gun and lots of flux. Took me some hours.
Second, I somehow managed to delete the two holes next to the display in the inner layer of acrylic, which are needed to fit the pins of the battery holder, when it's soldered on that side. I'm using new batch of battery holders, that have actual pins, and not wires, and they have the pins reversed compared to the previous batch, so they have to be soldered on that side.
In any case, it all fits together and holds nicely, and this time the buttons seem to be high enough. The cut-out for the USB plug doesn't look too bad, but the hanger part is quite fragile in the 1mm outer acrylic layer — I might modify the design to remove it from that layer, and only have it in the PCB and inner acrylic.
I'm still considering using a different piezo speaker — an SMD part, HYG7525 (aka XNQG 7525). It would go somewhere near the edge of the PCB, and it would have its own cout-out in the inner acrylic layer. It's exactly 2.5mm high, and the hole is on the side, so no cut-out in the outer layer would be necessary. It would make the assembly a little bit easier.
After the weekend, I'm going to publish the design files for version 4, because I think it's good enough to use. I'm going to build a few of those, and we will see how they work in the longer run — there might still be version 5.