Aluminum...
The purpose of backing plates or washers is to distibute the load across a greater area of the deck, hull, or whatever you are mounting hardware too. Oversized washers are often adequate. Aluminum is going to work just fine for most applications if you are going to through bolt it and in many cases when you are tapping it to receive a bolt. Hunter inbeds aluminum plate into the deck to mount hardware and they drill/tap to receive SS bolts.Al. is much easier to work with and less expensive than SS. You can find Aluminum backing plate stock in the BoatUS and West Marine catalogs, too.More info avail in link.Fair winds,Tom