Centering tool to slot 6mm axles or M6 screws by handsaw

The Screw of the X Axis Tensioner of my new Prusa i3 Hephestos was hard to turn, I wanted something better to grip. So I made this little Tool to guide the Saw Blade to slot the Screw.

The STL is made for 6mm (M6), but you can edit the parametric SCAD File.