To fix this problem with the leftover holes on a 2010+ you will need:
2x turn signal washers (Kawasaki P/N #92200-0165)
2x 8mm stainless steel locknuts
2x red rear reflectors removed during the installation of the side racks with 8mm nuts still in position
Remove the tail bracket assembly for ease of installation
Drill the holes in the washers to fit the shafts of the reflectors
Secure the reflector to the hole using the washer and locknut
(don't tighten too much or you may damage
the reflector's mounting post)
The radius of the reflector is sufficient to fit flat against the plastic, the shafts of the reflectors are just long enough for a secure mount, it covers the hole completely, and makes it look like it was designed that way.
Total cost? <$6 for the 4 pieces of hardware not already in possession and about 30 minutes of installation time.