This is a part designed to clamp down a 6mm shaft. The mounting holes are M4 (4mm diameter) and are spaced 30mm from eachother. The tensioning hole requires an M3x15 (or longer) bolt and nut. I designed this part in Autodesk Inventor 2017 (hence the .ipt file).
If you have any questions, feel free to ask!
I've designed this so that no supports are needed when laid flat with the tensioning hole horizontal to the bed. The tensioning hole has a droplet shape to prevent the need for supports.