RepRack M3 clamp for NA 1/2" EMT

This is a very quick remix of JakJak's M3 clamp for RepRack, sized down for North American 18mm / 1/2" EMT conduit. I didn't even bother modeling it in Fusion or OpenSCAD, this was created with modifications in Prusaslicer directly, and it worked well enough for me. I had originally tried scaling down the original model by ~3% but found the mounting holes to be out of alignment with the RepRack by enough to be uncomfortable. This remix respects the original dimensions but with a smaller opening for the pipe.