Hmmmm...yes I remember getting the same result when I looked into this subect for my Carby SS. I was told it 'might' be possible to use a universal type extenda but results would vairy.
I've stuck with with standard for the mean while but a good option was (I think) from:
http://www.ricambi-weiss.de/shop/cat...roducts_id=183 Sorry - they dont seem to do it anymore - they did a Carbon Fibre Fr mudguard that was much longer than the original, good price too as I remember.
Might be worth dropping them a line.
Cheers - Frank