You can try to use the Nanowire custom builder, and adjust the relative surface energies. Or, you can wait for ATK 12.2 for which we may (may!) have a beta-version ready for public release next week. There you can just cleave the Si, make a large supercell, then draw a rectangular shape with the mouse are cut it out. See the screenshot - this system takes a minute or less to build in VNL 12.2, and that's including the hydrogen passivation (click a button - done!).