Aluminum may be easier to work with than stainless steel but it will be a potential cause of corrosion due to dissimilar metals. For a part that small and the amount of effort to make it, I'd go with 316 stainless, 3/16" thick should do it. Just cut a rectangle, drill the holes and bolt it down. If you want to get fancy you could round the edges, but really, if you make it big enough (say, a width of 15 to 20 times the bolt hole diameter), the edges won't have any load so shape won't matter. I have heard G10 makes a good backing plate but it would have to be thicker, and I would prefer steel for a potentially large fluctuating load like a rudder. My main concern with G10 is creep, the tendency for a material to take a "set" or plasticly deform under load, combined with the cycling load, could cause the bolt to loosen over time. Preload on the bolt is the best resistance to loosening and material creep reduces preload. A good locking nut will help but I like extra insurance for critical functions.
Oh, and I used stainless backing plates when I installed new winches on my boat.