Use the user grid. (Or at least a very coarse grid setting)
Or you could use freecad + stepup if you have a more complex outline. (or dxf import or python scripting or KiCommand or …)
Links:
- StepUp: Kicad StepUp: The Sketcher for Getting to Blinky
- KiCommand: KiCommand discussion and development - Easy pcbnew command line; 4.07 & 5.1.5/.6
- DXF import (there are multiple threads here. This one is just an example) Import DXF with arcs from OpenSCAD, FreeCAD or LibreCAD into KiCAD