The Arduino stacking connectors have arrived, and, while rummaging through one of my boxes of parts, I also found a small quantity of 8-pin female headers, so I can now make four full kits. I tested the 8-pin headers and they work very well with the Pololu drivers (tight grip and good electrical contact, unchanged after a few dozen insertions). I will email the people who already bought the bare PCBs to see whether they may want the parts to go with their boards, and if there are any kits left after that, they’ll go in the store.