These are the track slugs for the Native Groove track system that are used in at least the following kayaks:
Native Slayer 10
Native Slayer 13
It is designed to hold a stainless steel 1/4" 20 and 1/4" 32 nut (note: hold, not grip) or their respective hex head bolts/screws.
Sizes are not exact (measured 5mm is programmed in scad as 6mm) as my printer (ROBO 3D) seems to have accuracy issues but feel free to edit the values in the scad files.