These Grayston Mud Flaps are a basic necessity in Stage Rallying - these are the ones that meet regulation 48.1.12. (Which requires a 4mm thick material). Don't be confused by cheaper 3mm options which do not meet the specification.
 
These come as 2 sheets per pack, each sheet measuring 500x300x4mm - often possible to get 2 mudflaps from one sheet.