Aluminium Extrusion T-Slot Profile 3030 3mm/4mm With Tapered Hole
Aluminum Extrusion T-Slot Profile 3030
Two designs with M3 (3mm) screw hole or M4 (4mm) screw hole.
Holes are slightly tapered (diameter gets narrower as you screw in deeper) so as to tighten as you screen in. Also expands the unit to lock it snugly in place.
See my other design for the flat version.