I used the Dremel with a small sander drum on my sks chromoplastics and one of the metal nibbling bits. For finer work, some sandpaper wrapped around a dowel is better.
You need to shape the two cutouts accurately so it still has some grip outwards to the chsinstays and can hold itself in place. I used a small ziptie through a hole 5mm from the tip to keep the fender in place.