I've use one of these:

http://www.ebay.co.uk/itm/MUD-FLAP-PRE-65-CLASSIC-TRIALS-MOTOX-ENDURO-SHERCO-GASGAS-4RT-BETA-EVO-etc-/400867019721

Alternatively you can buy mudflap material and cut your own:

http://www.ebay.co.uk/itm/Grayston-500mm-x-300mm-x-3mm-Mudflap-Material-Rally-Race-Rallycross-/182076817937?hash=item2a649fc211:g:jpcAAOSwP~tW1-Qa

__________________