I made one almost identical to the pic above from a sheet of thick rubber. It keeps a lot muck off the front cover. Don't expect it to be 100% successful though. The only mud guards that are truly effective are the really old fashioned valanced ones that wrap around the sides of the tire.
My bike came with the BMW rear mudflap which is quite substantial and moulded to the correct profile. I've never ridden the bike without it so I don't know how effective it is.