If you are thinking about using the chainsaw file to form the round in the depth of the slot? You could consider just drilling the closest sized hole there on a drill press. Then form the rest of the slot with what ever hacksaw, flat filing that would get you there. Maybe leave a little extra for work holding, that piece may be frustrating with a lot of vibration if using hand tools.
The others that commented would know much better, but Id drill the stop hole, hog it out with a disk dremel type tool, then just clean up the relatively lesser roughness. And, Im sorry if Im not understanding what youre trying to do. Best of luck with it.
