Utility Bed Rail - T-Slot nut size - M12 16mm slot
Summary: M12 T-Slot Nut for a 16mm slot ground down ~1-2mm in height. Only found these available at Mcmaster-Carr. 16mm length bolts are perfect. 25mm would likely be max when you add on extras to avoid bottoming the bolt out. If I were to do this again, I'd just cut metal plate into squares...
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