The Robot Friend boards just arrived from @oshpark a few minutes ago. All the holes and headers aligned perfectly and the board passed the electrical tests. 2.5M 8mm nylon hex screws and spacers from the Adafruit 2.5M nylon screw collection (https://www.adafruit.com/product/3299) were used to secure the boards.
The board uses two sets of standard-length Feather header pins. The hex spacers used between the boards should be 9mm, but those are difficult to find. then slid the header pin black plastic carrier strip down about 1mm to shorten the exposed (non-soldered) portion of the pins. The alternative would be to remove 1mm from some 2.5M 10mm nylon spacers, of course.
DigiKey carries a 2.5M 9mm metal hex spacer that I'll test after their order arrives. There will be a link to it posted here if it works.
May make a few mechanical adjustments to the final version to expose as many Crickit connectors as possible.