I have completed 8 bit planes, ready to assemble into a 8 x 8 x 8 bit (64 byte) stack:

Just waiting on some standoffs, cables and headers. Here's a closeup view of a plane showing the drive line connectors (1mm pitch FFC) and the sense and inhibit connectors (2-pin JST):

Finally, a view of the underside of the bottom plane, showing the pin connectors that plug into the carrier board:

Meanwhile, I am workng on the schematic for the carrier board, on which will be mounted the core stack and the logic shown in the diagram in the project details.