This is simply a 100mm long (scale this dimension to suit your needs) 2040 extrusion (20mm x 40mm). After several attempts in FreeCAD to produce a fully defined sketch, this one was the best. The hole diameter is 4.2mm, the outer wall thickness is 1.8mm. A 5mmx0.8mm pitch tap may be needed to thread this hole to a 5mm screw. Alternately, I have another model of a 2040 T-slot with a 4.5mm screw hole that is more easily threaded using a 5mm screw. See my model of a T-slot nut for a nut to fit this T-slot. Print it vertically without support. You may want to print with a raft for better adhesion if necessary for your scaled length.