I'd use a file, but I have plenty available.
Requirements are:
1) Hold it
flat against the edge of the slot.
2) Use moderately light pressure. Don't bear down.
3) Don't file more strokes than necessary
A file, when held flat against a surface, acts like a woodworker's hand plane. High spots are the first to be removed, and the rest will be minimally affected.
I went into the shop, removed my Pro-Filer stone, and tried a couple of positions that could work.
I tried a 6" mill bastard file, and it was small enough to fit the slot, but not so small that it affects your ability to hold it flat. In France, I don't know if that cut (an American nomenclature) is available, and I don't envy your attempts to communicate that to a French shop keeper.
The first method to register a file flat against the edge would probably require a vice (use soft jaws), but it's easy to hold the file flat against the surface for work. You will be limited to very short strokes in your filing motion though. Note that it looks like I'm putting significant pressure against the slot - I'm not. It's just a little more than the pressure used by two fingers when taking a pulse on your wrist. My thumbs are just making sure that the file is maintained flat against the edge.
I then tried to maintain flatness using the file holding it at only one end, and it's easy enough to keep square. I was working on the side away from the camera in this shot (the edge near me), which seems to work better than pressing against the far edge. Once again, if you have a vice, that would help to hold the Pro-Filer steady. It tends to rock because of the rounded shape.
I've done tons of metal filing in my lifetime. Used thoughtfully, it is a fast way to remove burrs. Remember,
hold it flat against the material, and use a light touch. Just a few passes will take down most burrs in aluminum.