Production spec
The daughterboard thickness should be 1.6mm.
Soldermask color, FR4 substrate color, silkscreen color, copper finish color can be chosen at will.
If aesthetic choices are modified, do make sure that the circuitry revision (i.e. C1, C3) are clearly indicated, and that aesthetics do not compromise the functionality of the board.
If circuitry is changed, it should be merged into this repository under a different name.
Cable considerations
A one-to-one JST-SH 4 pin cable is used to connect the daughterboard to the mainboard.
Make sure that the cable is properly one-to-one; a cable which inverts the pinout will result in reverse voltage applied to the main PCB resulting in its destruction.
One good option for testing and low quantity boards is the Sparkfun Qwiic cables; these are confirmed functional and compatible with the Unified USB Daughterboard series.
Digikey carries both the "correct" and "incorrect" variants of the JST cables in various lengths; before ordering, check the specification/drawing PDF for the item and verify that the cable is of the correct type.
The correct type is the 4-wire variant of the "Socket to Socket, Reversed" type cables listed here.
For higher quantity production, consider ordering custom cables from a cable factory.