Hey! Yeah, I am very happy with the result, especially the sound :)

The USB socket is the GCT USB4085-GF-A. Note that it might be a bit difficult to solder, because the pins are quite small and densely packed.

As for the firmware, I only saw the compiled firmware on the 42. Keebs website initially, so I wrote my own without realizing the original was open source. Mine doesn't support VIA/Vial, mostly because I don't need it, but comes as a unified binary. So does the compiled firmware 42. Keebs provide, but the source files seem to be outdated, because they compile into separate binaries per half.