You can always stick some masking tape on the slot sides, which should offer enough protection. If you start to wear it away, simply replace it. Or if you've got an old feeler gauge set you no longer use because its gone a bit rusty, then you could hold one of the thin feelers against the front of the slot.

Ideally, you'd go and get a suitable sized needle file, then grind the side edges off so that they're smooth and then dedicate the file to nut slots.