Hmm, interesting comment from RN. I bought mine from RN and it came with all the parts as listed in this instruction manual from the RN website, including the backing plates shown. I installed it last year and IMO, the backing plates are required. That said, if you can’t find them they should be easy to fabricate. Here’s a link to the RN instructions and parts list and a picture of mine installed.
http://c0007716.cdn2.cloudfiles.rackspacecloud.com/instructions/RNA2550I.pdf
http://c0007716.cdn2.cloudfiles.rackspacecloud.com/instructions/RNA2550I.pdf