Unbreakable plastic flap to go under the rear mudguard , stops all muck getting into the airbox. Longer and better and completely different from the Honda one!
 
Made by Mitani Japan to the same design as the carbon fibre one, only stronger and...just works! Yes they are expensive, but they do work. ( A Genuine Honda one is now £65!) 
2014 - 2019- All models. (will fit 2009 on but this sits better on new shaped 2014 onwards mudguard)
 
"Colour Black plate thickness 3mm As you know, Kyduck is the best material that has both light weight and high impact resistance. Making use of this characteristic, a rear fender stabilizer is manufactured.
Even if it is bent considerably, it is a flexible material that does not crack and lasts a long time. From the inside of the rear fender, the load of the rear fender is received and the strength is increased!
In addition, since it curves so that it closely adheres along the rear fender, it prevents the infiltration of mud and water into the air BOX! Factory rider item! 
When you want to add weight to the back on slippery surfaces, you can sit on the rear fender and put weight on the R tires when you muddy or stack. If there is no stabilizer, the rear fender will bend and you will not be able to apply weight, but it is okay if you have this stabilizer. As expected, the idea of ​​the factory is wonderful!"
 
A customer review - "I reckon riders accept that crap just gets in. Well it doesn’t with this. OEM is 40 odd quid and trick bits is junk... doesn’t take much thinking about!"