If you use a small enough disk you should be able to cut a slot without touching the chassis. Just need to take your time when cutting the slot.
If the cutting disk is round and a small enough diameter then the angle of the disk sides should be roughly the same angle as the countersink. You shouldn't then need to damage the chassis.
You could test the theory and practice on a similar screw in a countersunk piece of mdf if you are really worried about chassis damage.
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