Have trimmed lotsa fenders, most recently several Planet Bike models. Try mounting fenders on recumbents without being creative.
Side-cutting diagonal pliers are good for making cuts at the edges. Snips or shears can work to make controlled cuts across the fender.
Just a couple mm at the edges? Sandpaper? File? Rotary sanding wheel on your Dremel? Or whittle the edges down with a sharp knife blade?