This might be a dead end, but they have them for up to 2013 models here: http://grantkustoms.com/catalog/fillers-3rd-brake-fillers-chevygmc-fullsize-3rd-brake-light-filler-p-948.html
In the worst case, maybe you can find a local machine shop that will make a blank plate for you based on the profile of the 3rd brake light housing.