It is easy to fit & is adjustable. More importantly it reduces the amount of spray thrown up by the rear tyre & comes with a wunderlich metal sticker emblem or you can fit an identical size
BMW emblem that is available from BMW dealers.
Make one from a piece of 1mm black acrilyc sheet from a model shop £0.80. make it the same width as your # plate then using the # plate fixings drill it in the required position leaving 150-200mm below the plate, Simple.
In these times of ever increasing Gatso's,why on earth would anyone want to keep their number plate clean?
Anyway if you want one,buy a cheap paint roller tray,cut to the size of your number plate-leaving the sloping angled bit as the extenda,yours for ooooooh I dunno,a pound?