-
I'd use a rotary burr in the Dremel® for exactly the reason I wouldn't use a hand file - lower tool pressure from the Dremel® gets around the work holding problem.
Maybe, that's how I slotted the last one I did - still had to clamp the 'guard to stop it skittering away.
I'd clamp the file in the vice, then run the mudguard back and forth over the file, rather than filing the 'guard, if that makes sense?
I'd use a file for that, simply because clamping the mudguard would be a massive pain in the arse.