You can always just cut a piece of aluminum in about .080" for a backing plate. You'd have to really whack something hard to bend it at that thick. Tractor Supply sells it in 1' x 2' pieces for like $9.

When I make my "HEMI" fairlead plate covers I get my aluminum from the scrap pile at a local shop and they just charge me what they'd get for it by weight from the scrap yard. I can get a piece big enough for two plates for about a buck.



You can add a cheap plastic frame for about $1.99



or get a nicer stainless one for $5.