Creating a consistent template for routing the channels, such that the pre-built PDBs will fit perfectly into the grooves, was a real pain, so we've come up with a simple router jig, out of offcuts of wood and scrap metal runners.
The rails are placed parallel to each other, at exactly 420mm apart (in truth, we used a piece of 420mm square cut wood to line up the rails, only to find they're about 420mm apart at one end and 418mm apart at the other, occasionally binding on the wood as it passes through!) But it's good enough for a first try.
The base is marked at 35mm intervals, so that we simply push our board through, under the router, then line up the leading edge with the marks. Set the router head spinning, plunge, then run the router from left to right. As our cutting head is 6mm wide, and the plunge depth set to 3mm, this makes 12 channels for our hall sensors to sit in.
Then we rotate the wood through 90 degrees and use a 16mm cutting head to create wider grooves for the PCBs at 70mm apart (though in the image above, our first board was cut exclusively with a 6mm bit, making the wider grooves with multiple offset passes).
We made a bit of a mess of the first offset (at both ends) for the PCB channel - but once we'd worked it out, the rest of the channels were cut quickly and easily.
The end result is a piece of MDF into which we can easily drop our pre-populated circuit boards. In practice, it may be easier to connect the wires between the boards before fitting (as we did with our earlier jig-based soldering) but we're still ironing out the exact manufacturing process!
The nice thing about this process is that the boards can all be routed independently of the circuit boards, which are then fitted quickly and easily into the grooves. All that then remains is to fit the main controller (that is used to connect multiple boards to each other) on the underside, and a thin cardboard layer fitted to the top, to cover the currently exposed circuit boards.