 
A Chain Fin helps prevent your fingers and toes from getting caught between the chain and rear sprocket. This no frills Chain Fin sells for $20 each including hardware. They are unfinished aluminum and can be polished or painted and installed by you.
The bike shown is a 250 Ninja. The Chain Fin has been installed just in front of the rear stand spool. To install you drill two ¼ inch holes in the bottom of your swing arm and bolt the fin on using the included hardware.