there is no dedicated arch extension that I can find so I have got some 6mm conveyor rubber which I am going to screw under the arches somehow.
on my old terrano I bought the plastic strips which also screw up under the arches.
This is the stuff I have got to make the wheel arch extensions.
Just got to make some clamping straps so it follows the shape of the arch when fitted with no saggy floppy bits in between the fasteners.